High-Yield Preclinical Microbiology • COMLEX & USMLE

Clinical Microbiology & Infectious Disease

Dichotomous Gram-stain decision trees, bacterial exotoxin mechanisms (cAMP, EF-2, SNARE cleavage), viral BOAR and Baltimore taxonomy, dimorphic fungi, and classic parasitic board vignette clues.

Clinical Microbiology & Infectious Disease Question Engine COMLEX • USMLE • COMAT

Direct board practice questions covering organism taxonomy, antimicrobial stewardship, infection control, and osteopathic correlates.

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Preclinical Microbiology Suite Bacteriology • Virology • Mycology

Staph Subtypes, Listeria, Brucella, HPV Oncoproteins, Hep B Serology & Dimorphic Fungi

Coagulase-Negative Staphylococci & Listeria

• Staphylococcus epidermidis: Catalase-positive, coagulase-negative, Novobiocin sensitive. Normal skin flora that produces copious extracellular polysaccharide biofilms on prosthetic heart valves, orthopedic joint hardware, and central venous catheters.

• Staphylococcus saprophyticus: Catalase-positive, coagulase-negative, Novobiocin resistant. Second most common cause of uncomplicated community-acquired urinary tract infections in sexually active young women ("honeymoon cystitis").

• Listeria monocytogenes: Gram-positive facultative intracellular bacillus exhibiting distinctive tumbling motility at 22°C and survival at refrigeration temperatures (4°C; cold growth in unpasteurized milk, deli meats). Spreads cell-to-cell via host actin rockets (actin polymerization via ActA protein). Classic cause of neonatal meningitis and granulomatosis infantiseptica; meningitis in elderly/immunocompromised. Empiric ampicillin is mandatory.

Zoonotic Brucella & Papillomavirus (HPV)

• Brucella species: Zoonotic Gram-negative coccobacillus transmitted via contact with contaminated livestock or ingestion of unpasteurized dairy products (imported goat cheese). Causes undulant fever (rising and falling fever spikes), night sweats, hepatosplenomegaly, and osteomyelitis.

• Human Papillomavirus (HPV): Non-enveloped dsDNA virus.
– High-Risk Types 16 & 18: Produce oncoproteins E6 (binds and facilitates ubiquitin-mediated degradation of tumor suppressor p53) and E7 (binds and hyperphosphorylates/inactivates Rb, releasing E2F transcription factor → uncontrolled G1-S cell cycle transition). Etiologic agent of cervical, vulvar, penile, anal, and oropharyngeal squamous cell carcinomas.
– Low-Risk Types 6 & 11: Cause condylomata acuminata (anogenital warts) and recurrent respiratory papillomatosis.

Hepatitis B Serology Master Decoder

HBsAg:Surface Antigen

Indicates active current infection (acute or chronic if persistent >6 months).

Anti-HBs (HBsAb):Surface Antibody

Indicates long-term immunity (via resolved infection or vaccination).

Anti-HBc / HBcAb (IgM vs. IgG):Core Antibody

IgM = acute infection; sole positive marker during the window period. IgG = prior exposure. NEVER present in vaccinated individuals.

HBeAg / Anti-HBe:Envelope Marker

HBeAg indicates high viral replication and high infectivity; Anti-HBe indicates low transmissibility.

Systemic Endemic Dimorphic Fungi (Mold in the Cold 20°C, Yeast in the Beast 37°C)

Histoplasmosis

Mississippi & Ohio River valleys. Bird/bat droppings (spelunking, chicken coops). Microscopic: tiny yeasts inside macrophages. Mimics TB with apical granulomas & calcifications.

Blastomycosis

Great Lakes & Ohio River basin. Microscopic: large yeasts with single broad-based budding (same size as RBC). Causes pneumonia, verrucous skin lesions, and osteomyelitis.

Coccidioidomycosis

Southwestern US (California, Arizona; San Joaquin Valley). Dust storms, earthquakes. Microscopic: large spherules filled with endospores. Erythema nodosum, arthralgias (desert rheumatism).

Paracoccidioidomycosis

Latin America (Brazil). Microscopic: central yeast with multiple peripheral daughter buds resembling a "captain's wheel" or "Mickey Mouse" head. Mucocutaneous ulcers.

Diagnostic Algorithms & Morphology

Bacterial Diagnostic Decision Trees

Branching diagnostic algorithms for Gram-positive cocci, bacilli, and branching filaments, plus Gram-negative rods, diplococci, and curved organisms.

Master Algorithm Cocci • Bacilli • Branching

Gram-Positive Bacteria Diagnostic Algorithm

Comprehensive diagnostic branching algorithm mapping Catalase, Coagulase, Hemolysis, Novobiocin, Bacitracin, Optochin, spore formation, and branching acid-fast patterns.

Board Pearl: S. epidermidis is Novobiocin SENSITIVE (biofilms on hardware); S. saprophyticus is Novobiocin RESISTANT (UTIs). S. pneumoniae is Optochin SENSITIVE (bile soluble); Viridans is Optochin RESISTANT.

G+

Gram-Positive Cocci Decision Tree

Catalase → Coagulase → Hemolysis → Antibiotic Disc Sensitivities

Catalase (+) • Coagulase (+) β-Hemolytic

Staphylococcus aureus

Clusters ("grapes"). Ferments mannitol on mannitol salt agar (yellow colonies).

Key Virulence: Protein A (binds Fc portion of IgG, prevents opsonization and phagocytosis). Coagulase forms fibrin clot around organism.

Diseases: Acute infective endocarditis (tricuspid in IVDU), osteomyelitis (most common cause overall), septic arthritis, bullous impetigo, toxic shock syndrome (TSST-1), scalded skin syndrome (exfoliatin), rapid-onset food poisoning (preformed enterotoxin in mayo/custards, 2-6 hrs).

MRSA: Altered PBP (PBP2a encoded by mecA gene) → resistant to all β-lactams. Treat with Vancomycin or Daptomycin.

Catalase (+) • Coagulase (-) γ-Hemolytic

Staph epidermidis vs Saprophyticus

Novobiocin sensitivity test differentiates these two coagulase-negative staphs.

S. epidermidis (Novobiocin SENSITIVE) Biofilm-producer (glycocalyx/slime layer). Infects prosthetic heart valves, orthopedic joint hardware, intravenous catheters, and VP shunts. High resistance; treat with Vancomycin.

S. saprophyticus (Novobiocin RESISTANT) Normal flora of female genital tract and perineum. Second most common cause of uncomplicated community-acquired UTI in sexually active young women (after E. coli).

Mnemonic: "NO Staph (NOvobiocin = Saprophyticus resistant, Epidermidis sensitive)"

Catalase (-) • β-Hemolytic Bacitracin SENSITIVE

Streptococcus pyogenes (Group A)

Pyrrolidonyl arylamidase (PYR) positive. Chains of cocci. Hyaluronic acid capsule (antiphagocytic, non-immunogenic).

M Protein Virulence: Master antiphagocytic fibrillar surface protein; binds factor H to inhibit alternative complement pathway. Epitope mimicry against human cardiac myosin triggers Acute Rheumatic Fever (Jones criteria: Migratory polyarthritis, Carditis, Subcutaneous nodules, Erythema marginatum, Sydenham chorea).

Enzymes & Diagnostic Serology:

Streptolysin O: Oxygen-labile pore-forming hemolysin. ASO titer confirms antecedent pharyngitis (rheumatic fever/PSGN).
DNase B (Streptodornase): Cleaves extracellular DNA nets. Anti-DNase B titer is the most sensitive marker for antecedent GAS impetigo/skin infection leading to PSGN!
Streptococcal Pyrogenic Exotoxins (SpeA, SpeC): Phage-encoded superantigens crosslinking MHC-II to TCR Vβ → massive IL-1/IL-2/IFN-γ/TNF-α storm → Scarlet fever (sandpaper rash, strawberry tongue, circumoral pallor) and Toxic Shock-Like Syndrome (TSLS). SpeB protease drives necrotizing fasciitis.

Critical Board Sequela Rule: Early Penicillin G/VK therapy prevents Rheumatic Heart Disease, but does NOT prevent Post-Streptococcal Glomerulonephritis (PSGN)! Rheumatic fever follows pharyngitis ONLY; PSGN can follow either pharyngitis OR skin impetigo.

Catalase (-) • β-Hemolytic Bacitracin RESISTANT

Streptococcus agalactiae (Group B)

Polysaccharide capsule (antiphagocytic). Narrow zone of β-hemolysis.

CAMP Factor & Hippurate Positive: Secretes CAMP factor (diffusible protein that enlarges area of hemolysis produced by β-lysin of S. aureus, creating pathognomonic arrowhead hemolysis). Hippurate test positive (hydrolyzes sodium hippurate to glycine and benzoic acid).

Clinical Focus: #1 leading cause of neonatal sepsis, neonatal meningitis, and neonatal pneumonia (vertical transmission during vaginal delivery).

Screening & Prophylaxis: All pregnant women screened at 36 0/7 to 37 6/7 weeks gestation via rectovaginal swab. Intrapartum IV Penicillin G or Ampicillin administered during labor if culture-positive.

Mnemonic: "B for Babies, Bacitracin-resistant, CAMP arrowhead".

Catalase (-) • α-Hemolytic Optochin SENSITIVE

Streptococcus pneumoniae

Lancet-shaped diplococci. Bile soluble. Polysaccharide capsule (Quellung reaction positive).

Diseases (MOPS): #1 cause of Meningitis (adults), Otitis media (children), Pneumonia (typical, lobar, "rusty" sputum), Sinusitis.

IgA Protease: Cleaves secretory IgA to colonize respiratory mucosa.

Splenectomy Risk: Encapsulated; asplenic patients (sickle cell disease) suffer catastrophic septic shock without vaccination (PCV13 / PPSV23).

Catalase (-) • α/γ-Hemolytic Optochin RESISTANT

Viridans Strep & Enterococci

Insoluble in bile. Differentiated by growth in 6.5% NaCl and bile esculin agar.

Strep mutans, mitis & sanguinis: Normal commensal oropharyngeal flora. S. sanguinis synthesizes sticky extracellular dextrans (glucans) from dietary sucrose via dextransucrase, facilitating tenacious adherence to fibrin-platelet aggregates on pre-damaged heart valves → Subacute Bacterial Endocarditis (SBE) following dental procedures.

Enterococcus (faecalis / faecium): Grows in 6.5% NaCl and 40% bile (PYR+). Causes biliary tract infections, nosocomial UTIs, and subacute endocarditis after GI/GU instrumentation. VRE encodes vanA gene (D-Ala-D-Lac cell-wall peptidoglycan substitution, preventing vancomycin binding).

Strep gallolyticus (bovis): Grows in bile but NOT 6.5% NaCl. Associated with occult colorectal adenocarcinoma when presenting as bacteremia or native valve endocarditis (requires immediate colonoscopy)!

G+

Gram-Positive Bacilli & Branching Rods

Spore-Forming Aerobes vs Anaerobes, Non-Spore Bacilli, and Filamentous Branching Actinomycetes

Spore-Forming Aerobic Rods Poly-D-Glutamate Capsule

Bacillus anthracis & cereus

Large boxcar-shaped rods in chains. Anthracis capsule is unique (poly-D-glutamic acid polypeptide, NOT polysaccharide!).

Bacillus anthracis (Anthrax)

Cutaneous: Painless black necrotic eschar with extensive non-pitting gelatinous edema (Edema factor = adenylate cyclase; Lethal factor = zinc metalloprotease cleaving MAPK).
Inhalational (Woolsorter's): Inhaled spores from goat hides/wool → hemorrhagic mediastinitis, widened mediastinum on CXR, rapidly fatal shock.

Bacillus cereus (Food Poisoning)

Emetic form (cereulide): Preformed heat-stable toxin in reheated fried rice (nausea/vomiting within 1-5 hours).
Diarrheal form: Heat-labile enterotoxin formed in gut from contaminated meats/sauces (watery diarrhea at 8-18 hours).

Spore-Forming Anaerobic Rods Obligate Anaerobes

Clostridium tetani, botulinum, perfringens, difficile

Potent exotoxin producers. Spores survive boiling; killed by autoclaving.

C. tetani: Tetanospasmin cleaves synaptobrevin (SNARE) → blocks Renshaw cell GABA/glycine → spastic paralysis, trismus (lockjaw), risus sardonicus.

C. botulinum: Botulinum toxin cleaves SNARE → blocks stimulatory ACh release at NMJ → descending flaccid paralysis (canned foods in adults; raw honey in floppy infants).

C. perfringens: Alpha toxin (lecithinase / phospholipase C) → cell membrane breakdown, gas gangrene (myonecrosis with crepitus), double zone of hemolysis.

C. difficile: Toxin A (enterotoxin) + Toxin B (cytotoxin actin depolymerization) → pseudomembranous colitis post-antibiotics. Treat with oral Vancomycin or Fidaxomicin.

Non-Spore Rods & Branching Filaments Acid-Fast & Granules

Corynebacterium, Actinomyces, Nocardia

Club-shaped bacilli and true branching filamentous Gram-positive rods.

Corynebacterium diphtheriae: Club-shaped rods in "Chinese letter" arrangements. Metachromatic (babes-Ernst) volutin granules on Loeffler medium; black colonies on cystine-tellurite agar. Exotoxin inhibits EF-2 by ADP-ribosylation. Gray pseudomembrane, bull-neck, myocarditis, arrhythmias. Elek immunodiffusion test confirms toxin.

Actinomyces israelii (Anaerobe, NOT Acid-Fast): Normal oral flora. Causes cervicofacial "lumpy jaw" abscesses draining through sinus tracts with yellow "sulfur granules" (clumps of filaments) following dental extraction or trauma. Treat with high-dose Penicillin G.

Nocardia asteroides (Aerobe, Weakly Acid-Fast): Found in soil. Branching filamentous rod with mycolic acid (modified Kinyoun acid-fast positive). Pulmonary cavitary lesions mimicking TB in immunocompromised; metastasizes to create multiple brain abscesses. Treat with TMP-SMX.

Master Algorithm Diplococci • Coccobacilli • Enterics • Curved

Gram-Negative Bacteria Diagnostic Algorithm

Complete diagnostic flowchart for Gram-negative organisms: Maltose/Glucose fermentation, X & V factors, MacConkey lactose fermenters, Oxidase, H2S production, and motility.

Board Pearl: Neisseria meningitidis ferments BOTH Maltose and Glucose (M & G). Neisseria gonorrhoeae ferments Glucose only (G)! Salmonella produces H2S (black colonies); Shigella does not produce H2S.

G-

Gram-Negative Rods (Enterics & Respiratory)

MacConkey Lactose Fermentation → Oxidase → H2S Production

Lactose Fermenter (+) • Pink on MacConkey

E. coli & Klebsiella pneumoniae

Escherichia coli Green metallic sheen on EMB agar. Indole positive.

ETEC: Watery traveler's diarrhea (heat-labile LT stimulates cAMP; heat-stable ST stimulates cGMP → water/electrolyte secretion).
EPEC (Enteropathogenic): Adheres to apical enterocytes via bundle-forming pili, secretes Tir via type III secretion → intimin-mediated actin pedestal formation and effacement of brush-border microvilli → malabsorptive, non-bloody watery diarrhea in infants and pediatric nurseries.
EHEC (O157:H7): Shiga-like toxin (verotoxin, inactivates 60S ribosomal subunit by cleaving 28S rRNA), dysentery, HUS (triad: microangiopathic hemolytic anemia, thrombocytopenia, acute kidney injury). Does NOT ferment sorbitol!
UPEC: P-fimbriae (pili) adhere to urothelium → #1 cause of UTI & pyelonephritis.
Neonatal Meningitis: K1 capsular antigen!

Klebsiella pneumoniae Mucoid colonies (hypercapsulated). Urease positive. High risk in alcoholics and diabetics. "Currant jelly" dark red sputum, cavitation in right upper lobe.

Lactose Non-Fermenter • Oxidase (-)

Salmonella, Shigella, Proteus

Clear/white colonies on MacConkey. Differentiated by H2S on TSI agar and motility.

Salmonella enterica & typhi (H2S Positive - Black) Motile (flagella), produce H2S. High infectious dose (acid-labile, destroyed by stomach acid). Osteomyelitis in sickle cell disease! S. typhi causes typhoid fever: rose spots, step-ladder fever, pulse-temperature dissociation, gallbladder carrier state.

Shigella dysenteriae / flexneri (H2S Negative) Non-motile, acid-stable (extremely low infectious dose: ~10 organisms). Invades M cells of Peyer's patches. Actin polymerizer for cell-to-cell spread. Shiga toxin causes HUS.

Proteus mirabilis (Swarming Motility) Urease positive → splits urea to NH3 and CO2 → elevates urine pH (>8.0) → precipitates struvite / magnesium ammonium phosphate staghorn calculi.

Lactose Non-Fermenter • Oxidase (+)

Pseudomonas aeruginosa

Obligate aerobe, non-lactose fermenter. Grape-like sweet fruity odor. Produces blue-green pyocyanin/pyoverdine pigment.

Exotoxin A: Inactivates elongation factor 2 (EF-2) by ADP-ribosylation, halting host cell protein synthesis (identical to Diphtheria toxin!).

Mnemonic PSEUDOMONAS:

Pneumonia (cystic fibrosis, ventilated)

Sepsis (black necrotic skin: ecthyma gangrenosum)

External otitis (swimmer's ear / malignant in elderly DM)

UTIs (indwelling Foley catheters)

Drug use / Diabetic osteomyelitis

Osteochondritis (stepping on nail through rubber sneakers!)

Hot tub folliculitis, Burn patient wound infections

Enteric Rods & AmpC Producers

Enterobacter, Serratia, Yersinia

Nosocomial invaders and cold-resistant zoonotic enterics.

Serratia marcescens Motile slow lactose fermenter. Produces characteristic bright red pigment (prodigiosin) on room temperature culture. Common cause of hospital-acquired UTIs, catheter-associated bacteremia, and endocarditis in IV drug users.

Enterobacter cloacae & aerogenes Lactose fermenter. Harbor inducible chromosomal AmpC β-lactamase → resistant to cephalosporins (requires Cefepime or Carbapenems). Hospital-acquired pneumonia and UTIs.

Yersinia enterocolitica (Pseudoappendicitis) Transmitted by contaminated pork (chitterlings), pet feces, or unpasteurized milk. Grows at cold refrigeration temperatures (4°C). Causes mesenteric adenitis in children → mimics acute appendicitis (right lower quadrant pain + diarrhea). Can trigger reactive arthritis.

Oxidase (+) • Comma & Curved Bacilli

Campylobacter, Vibrio, H. pylori

Characteristic morphology: curved rods, comma shapes, and S-shapes.

Campylobacter jejuni (42°C Thermophile) Curved, gull-wing shaped Gram-negative rod. Oxidase (+). Grows at high temperatures (42°C) on Campy blood agar. Leading cause of acute bacterial diarrhea in US (poultry, unpasteurized milk). High-yield complication: Guillain-Barré syndrome (molecular mimicry to GM1 gangliosides) and Reactive Arthritis.

Vibrio cholerae & Vibrio vulnificus Comma-shaped rods, alkaline environment lovers (TCBS agar). V. cholerae: rice-water stool via Gs permanent ADP-ribosylation. V. vulnificus: raw oysters, saltwater wounds → fulminant bullous skin lesions, necrotizing fasciitis, and shock in cirrhotics / hemochromatosis.

Helicobacter pylori (Urease Champion & Oncogenic Cytotoxins) Curved Gram-negative rod colonizing gastric antrum. Strong Urease producer (converts urea to ammonia & CO2 → alkaline microenvironment shields from gastric acid; positive urea breath test).

• CagA (Cytotoxin-Associated Gene A): Injected into epithelial cells via type IV secretion system → disrupts host cytoskeletal architecture, drives severe inflammation, and dramatically elevates risk of gastric adenocarcinoma.

• VacA (Vacuolating Cytotoxin): Pore-forming toxin inducing pore formation in host mitochondrial membranes, pore swelling/vacuolation, and apoptosis.

• Neoplasia & Therapy: Chronic antral gastritis, duodenal and gastric peptic ulcers, gastric adenocarcinoma, and MALT lymphoma (regresses with H. pylori eradication!). Quadruple therapy: Bismuth subsalicylate + Metronidazole + Tetracycline + PPI.

Diplococci & Fastidious Coccobacilli

Neisseria, Haemophilus, Bordetella

Enriched medium requirements: Thayer-Martin and Chocolate agar.

Neisseria meningitidis vs Gonorrhoeae Both oxidase (+) diplococci grown on Thayer-Martin (VPN) agar. N. meningitidis: polysaccharide capsule, ferments Maltose + Glucose, causes epidemic meningitis and Waterhouse-Friderichsen adrenal hemorrhage. N. gonorrhoeae: NO capsule, ferments Glucose only, antigenic variation of pilus, urethritis/cervicitis, PID, asymmetric purulent septic arthritis.

Haemophilus influenzae Small pleomorphic coccobacillus. Requires Factor V (NAD+) and Factor X (hematin) on chocolate agar; "satellitism" around S. aureus colonies. Type b polyribosylribitol phosphate (PRP) capsule causes epiglottitis (cherry-red swollen epiglottis, "thumbprint" sign), meningitis, and otitis media.

Bordetella pertussis & Moraxella B. pertussis: Bordet-Gengou / Regan-Lowe agar. Pertussis toxin disables Gi → lymphocytosis, paroxysmal whooping cough with post-tussive emesis. Moraxella catarrhalis: normal flora causing otitis media and COPD exacerbations.

Spirochetes Master Reference (Treponema, Borrelia, Leptospira)

Spiral-shaped bacteria with axial filaments (endoflagella). Diagnostic staining, clinical staging, and board-favorite buzzwords.

Spirochete & Morphology	Transmission & Vector	Staging & Clinical Manifestations	Diagnostics & Treatment
Treponema pallidum Corkscrew motility, axial fibrils	Sexual contact, transplacental (TORCH)	Primary: Single painless hard chancre with indurated base. Secondary: Diffuse maculopapular rash on palms and soles, condyloma lata (broad flat lesions), moth-eaten alopecia. Tertiary: Gummas (chronic granulomatous lesions), thoracic aortic aneurysm (tree-bark aorta from endarteritis obliterans of vasa vasorum), Tabes dorsalis (dorsal column ataxia, Romberg +), Argyll Robertson pupil (accommodates but does not react to light). Congenital: Saber shins, saddle nose, Hutchinson notched incisors, mulberry molars, sensorineural deafness.	Darkfield microscopy for chancre fluid. Screening: VDRL/RPR (non-treponemal cardiolipin). Confirmatory: FTA-ABS. Rx: Benzathine Penicillin G. Watch for Jarisch-Herxheimer reaction (acute fever, chills, hypotension within hours of penicillin due to spirochete lysis and endotoxin release).
Borrelia burgdorferi Loosely coiled spirochete, Wright/Giemsa	Ixodes scapularis (deer tick); Northeast US	Stage 1 (Early localized): Erythema migrans (bulls-eye rash), flu-like illness. Stage 2 (Early disseminated): Bilateral Bell facial palsy, AV conduction block / myocarditis, migratory arthralgias. Stage 3 (Late persistent): Chronic asymmetric monoarthritis (knee), peripheral neuropathy, encephalopathy.	Two-tier serology: ELISA followed by Western blot. Rx: Doxycycline (Amoxicillin in pregnant patients or children <8). Ceftriaxone for heart block / CNS disease.
Leptospira interrogans Hook-ended "question mark" spirochete	Water contaminated with animal urine (rats, dogs, livestock); surfers, triathletes, sewage workers in Hawaii/tropics	Leptospirosis (Mild): Sudden flu-like illness, high fever, severe headache, prominent calf myalgias, and conjunctival suffusion (bilateral red eyes WITHOUT inflammatory exudate). Weil Disease (Severe icterohemorrhagic): Triad of profound jaundice / hepatic failure, acute kidney failure (azotemia), and hemorrhagic diathesis with pulmonary hemorrhage.	Darkfield microscopy, serology (microscopic agglutination test - MAT). Rx: Doxycycline (mild) or IV Penicillin G (severe Weil disease).

Zoonotic & Vector-Borne Infections

Tick vectors, animal reservoirs, distinctive rash morphologies, and first-line antimicrobial regimens.

Pathogen & Classification	Vector & Reservoir	Clinical Features & Classic Board Presentation	Diagnosis & First-Line Treatment
Borrelia burgdorferi (Spirochete)	Ixodes scapularis (deer tick); White-footed mouse (larvae), White-tailed deer (adult)	Stage 1: Erythema migrans (bulls-eye rash), flu-like illness. Stage 2: Bilateral Bell palsy, AV conduction block (carditis), migratory arthralgias. Stage 3: Chronic monoarthritis (knee), encephalopathy.	Doxycycline (or Amoxicillin in pregnant / kids <8). Ceftriaxone for heart block / CNS.
Babesia microti (Protozoan)	Ixodes scapularis (same deer tick vector as Lyme; co-infection is common!)	Hemolytic anemia, fever, jaundice, hemoglobinuria, hepatosplenomegaly. Asplenic patients have life-threatening fulminant disease.	Maltese cross / intraerythrocytic ring forms on Giemsa blood smear. Atovaquone + Azithromycin.
Anaplasma phagocytophilum	Ixodes scapularis; Northeast / Upper Midwest US	High fever, headache, severe myalgias, leukopenia, and thrombocytopenia. Rash is rare (<10%).	Morulae (mulberry-like cytoplasmic inclusions) inside granulocytes / neutrophils. Doxycycline.
Ehrlichia chaffeensis	Amblyomma americanum (Lone Star tick); South-Central / SE US	High fever, rigors, leukopenia, thrombocytopenia, elevated AST/ALT. Rash seen in ~30%.	Morulae inside monocytes. Mnemonic: MEGA (Monocytes = Ehrlichia, Granulocytes = Anaplasma). Doxycycline.
Rickettsia rickettsii (RMSF)	Dermacentor variabilis / andersoni (dog / wood tick); North Carolina, Oklahoma, SE	Severe headache, high fever, vasculitis. Rash starts peripherally on wrists and ankles and spreads centripetally to trunk. Involves palms and soles!	Endothelial tropism → necrotizing vasculitis. Treat immediately with Doxycycline (even in kids).
Francisella tularensis	Dermacentor ticks, deer flies, contact with wild rabbits / rodents	Ulceroglandular tularemia: painful necrotizing cutaneous ulcer with massive regional painful lymphadenopathy. Aerosol exposure causes fatal pneumonic tularemia.	Biosafety Level 3 pathogen (potential bioweapon). Treatment: Streptomycin or Gentamicin.
Bartonella henselae	Cat scratches / bites, cat fleas (Ctenocephalides)	Immunocompetent: Cat scratch disease (subacute regional lymphadenopathy, stellate necrotizing granulomas). Immunocompromised (HIV): Bacillary angiomatosis (vascular red-purple papules resembling Kaposi sarcoma) and peliosis hepatis.	Warthin-Starry silver stain positive. Cat scratch: Azithromycin; Bacillary angiomatosis: Doxycycline or Erythromycin.
Pasteurella multocida	Normal oral flora of cats and dogs	Rapidly progressing, excruciatingly painful cellulitis and wound necrosis within <24 hours of a cat/dog bite. Can cause rapid osteomyelitis.	Gram-negative coccobacillus with bipolar staining. Treatment: Amoxicillin-Clavulanate (Augmentin).
Yersinia pestis (Plague)	Rodent fleas (Xenopsylla cheopis), prairie dogs in Southwestern US	Bubonic plague: sudden fever, chills, weakness, and extremely painful, swollen suppurative regional lymph nodes (buboes in groin/axilla). Septicemic and pneumonic forms fatal.	Bipolar "safety-pin" staining on Giemsa/Wayson stain. Treatment: Gentamicin or Streptomycin.
Coxiella burnetii (Q Fever)	Aerosols from placenta / amniotic fluid of cattle, sheep, goats	Acute: high fever, debilitating headache, severe retro-orbital pain, atypical pneumonia, and hepatitis (doughnut granulomas). Chronic: culture-negative endocarditis in patients with pre-existing valvular disease.	Obligate intracellular bacterium with spore-like stability. NO arthropod vector! Treatment: Doxycycline.

Atypical, Cell-Wall-Deficient & Intracellular Pathogens

Pathogens that do not conform to standard Gram staining due to absence of cell wall, intracellular lifestyle, or minimal peptidoglycan.

Atypical • Intracellular Silver / BCYE Agar

Legionella pneumophila

Transmission: Aerosols from contaminated water sources (air conditioning cooling towers, hospital plumbing, hot tubs). No person-to-person transmission!

Presentation: Severe atypical pneumonia in elderly/smokers, high fever (>40°C), neurological symptoms (confusion, ataxia), watery diarrhea, and hyponatremia.

Diagnostic & Rx: Urine antigen test. Culture on Buffered Charcoal Yeast Extract (BCYE) agar supplemented with iron and L-cysteine. Rx: Levofloxacin or Azithromycin.

Cell-Wall Deficient Sterol Membrane

Mycoplasma pneumoniae

Structure: Completely lacks a peptidoglycan cell wall (naturally resistant to all β-lactams!). Outer membrane contains sterols sequestered from host.

Presentation: "Walking pneumonia" in military recruits, college dorms, young adults. Persistent nonproductive hacking cough, diffuse patchy bilateral interstitial infiltrates on CXR that look much worse than patient appears.

Hallmarks & Rx: High titer of Cold Agglutinins (IgM antibodies that agglutinate RBCs at 4°C); can precipitate hemolytic anemia and Erythema Multiforme (target lesions). Rx: Macrolides or Doxycycline.

Obligate Intracellular EB / RB Cycle

Chlamydia trachomatis

Biphasic Life Cycle: Elementary Body (EB) is infectious, "Enters" cell via endocytosis. Reticulate Body (RB) is metabolically active, "Replicates" via binary fission inside inclusions.

Serovar Subtypes:
• Serovars A, B, Ba, C: Trachoma (chronic follicular conjunctivitis leading to eyelid scarring and blindness).
• Serovars D–K: Urethritis, cervicitis, PID, ectopic pregnancy, reactive arthritis; neonatal staccato-cough pneumonia & conjunctivitis.
• Serovars L1, L2, L3: Lymphogranuloma venereum (LGV): painless genital ulcer followed by painful suppurative inguinal buboes with "groove sign".

Rx: Doxycycline (1st line) or single-dose Azithromycin. Treat co-infection with Ceftriaxone for N. gonorrhoeae.

Pathogenesis & Mechanisms ADP-Ribosylation • Superantigens • Neurotoxins

Bacterial Exotoxins & Cellular Mechanisms

Cellular mechanisms of major bacterial exotoxins: EF-2 inactivation, 60S ribosomal cleavage, Gs/Gi adenylate cyclase modulation, SNARE proteolysis, and MHC II/TCR crosslinking.

Board Pearl: Tetanospasmin blocks inhibitory neurotransmitters (GABA & Glycine from Renshaw cells) causing SPASTIC paralysis. Botulinum toxin blocks stimulatory neurotransmitter (ACh at NMJ) causing FLACCID paralysis!

Biochemical Virulence Mechanisms

Bacterial Exotoxins & Cellular Targets

High-yield COMLEX/USMLE pharmacology and pathology connections categorized by cellular mechanism of action.

Organism & Toxin	Molecular Target & Mechanism	Intracellular Effect	Clinical Manifestation & Board Pearls
Corynebacterium diphtheriae Diphtheria toxin	Inactivates Elongation Factor-2 (EF-2)	ADP-ribosylation halts host protein synthesis → cell death	Pseudomembranous pharyngitis (gray-white friable membrane), "bull neck" cervical lymphadenopathy, myocarditis with heart block, arrhythmias.
Pseudomonas aeruginosa Exotoxin A	Inactivates Elongation Factor-2 (EF-2)	ADP-ribosylation halts host protein synthesis → cell death	Host tissue necrosis, ecthyma gangrenosum (black necrotic bullae in neutropenic/leukemic patients), severe necrotizing pneumonia.
Vibrio cholerae Cholera toxin	Permanently activates Gs protein	↑ Adenylate cyclase → ↑↑ cAMP → ↑ Cl- & H2O secretion into gut lumen	Profuse painless "rice-water" diarrhea. Massive electrolyte loss, hypovolemic shock. Treat with aggressive oral rehydration solution (glucose + NaCl).
Enterotoxigenic E. coli (ETEC) Heat-labile (LT) & Heat-stable (ST)	LT: ↑ cAMP (Gs) ST: ↑ cGMP (Guanylate cyclase)	↑ cAMP and ↑ cGMP → ↑ Cl- and water efflux	Watery traveler's diarrhea. Mnemonic: "Labile in the Air (cAMP), Stable on the Ground (cGMP)".
Bordetella pertussis Pertussis toxin	Disables Gi protein (ADP-ribosylation)	Cannot turn OFF adenylate cyclase → ↑ cAMP → lymphocytosis	Whooping cough: catarrhal phase (coryza) → paroxysmal phase (inspiratory whoop, post-tussive emesis) → convalescent. Striking marked peripheral lymphocytosis.
Clostridium tetani Tetanospasmin	Cleaves SNARE (synaptobrevin)	Blocks inhibitory GABA and glycine release from Renshaw cells	Spastic paralysis, trismus (lockjaw), risus sardonicus (sinister grin), opisthotonos (arching of back). Retrograde axonal transport from puncture wound to spinal cord.
Clostridium botulinum Botulinum neurotoxin	Cleaves SNARE (synaptobrevin)	Blocks stimulatory Acetylcholine (ACh) release at neuromuscular junction	Flaccid descending paralysis: 4 D's (Diplopia, Dysarthria, Dysphagia, Dyspnea). Adults ingest preformed toxin in canned food; infants ingest spores in raw honey (floppy baby).
Staph aureus / Strep pyogenes TSST-1 & Exotoxin A (SpeA)	Superantigen: crosslinks β-chain of TCR to MHC Class II	Uncontrolled activation of up to 20% of T-cells → cytokine storm	Massive release of IL-1, IL-2, IFN-γ, TNF-α. Severe hypotension, diffuse macular erythroderma rash with desquamation of palms/soles, multiorgan failure. Associated with prolonged tampon use or nasal packing.

Acid-Fast Pathology TB Stages • Cord Factor • MAC • Leprosy

Mycobacteriology & Acid-Fast Diagnostics

Acid-fast staining, M. tuberculosis virulence factors (cord factor, sulfatides), primary vs. reactivation disease, Pott disease, and Tuberculoid (Th1) vs. Lepromatous (Th2) leprosy.

Board Pearl: Cord factor induces TNF-alpha release and granuloma maintenance. MAC causes disseminated disease in AIDS with CD4 < 50; prophylaxis with Azithromycin.

Acid-Fast Bacilli & Granulomatous Pathology

Mycobacteriology & Clinical Tuberculosis Suite

Virulence lipid biochemistry, PPD induration thresholds, RIPE toxicities, Hansen disease immunology, and atypical NTM infections.

Virulence Factor

Cord Factor (Trehalose Dimycolate)

Glycolipid component of the mycobacterial cell wall responsible for serpentine growth in parallel cords in vitro.

Pathophysiologic Action: Inhibits macrophage migration, disrupts mitochondrial respiration, and stimulates TNF-α release, inducing caseating granuloma formation.

Key Exam Clue: Serpentine cords on smear = virulent Mycobacterium tuberculosis strain!

Intracellular Survival

Sulfatides (Sulfolipids)

Sulfated trehalose glycolipids found on the mycobacterial cell envelope.

Pathophysiologic Action: Prevent phagolysosome fusion inside alveolar macrophages, allowing bacteria to multiply uninhibited within phagosomes.

Key Exam Clue: Acid-fast retention via Ziehl-Neelsen (carbolfuchsin resists acid-alcohol decolorization).

Pathology & Staging

Ghon Complex vs. Ranke Complex

Primary TB: Inhaled droplet seeds mid/lower lung lobe parenchyma → subpleural Ghon focus + ipsilateral hilar lymphadenopathy = Ghon complex.

Ranke Complex: Fibrosis and dystrophic calcification of the Ghon complex detectable on chest radiograph.

Reactivation TB: Occurs in apical/posterior upper lobes due to high local ventilation-perfusion (V/Q) ratio and high oxygen tension!

Tuberculin Skin Test (TST / PPD) Induration Threshold Matrix

Transverse diameter measurement of induration (NOT erythema) read at 48 to 72 hours.

Threshold	Target High-Risk Populations	Immunologic Rationale	Next Best Clinical Step
≥ 5 mm	• HIV-infected individuals • Recent close contacts of patients with active contagious TB • Persons with fibrotic changes on CXR consistent with prior inactive TB • Solid organ transplant recipients or patients on immunosuppression (prednisone ≥ 15 mg/day or TNF-α inhibitors)	Severely impaired cell-mediated immunity produces blunted cutaneous delayed-type hypersensitivity (DTH) reactions.	Order CXR. If clear: Treat Latent TB (Rifampin x 4 mo or INH+Rifapentine x 3 mo). If symptomatic/infiltrate: Sputum AFB x 3.
≥ 10 mm	• Recent immigrants (< 5 years) from high-prevalence TB countries • Injection drug users (IDU) • Residents/employees of high-risk congregate settings (prisons, nursing homes, healthcare facilities, homeless shelters) • Mycobacteriology laboratory personnel • Clinical conditions increasing TB reactivation risk (silicosis, diabetes, CKD, gastrectomy, leukemia) • Children < 4 years of age or infants exposed to high-risk adults	Moderate environmental exposure risk or clinical comorbidities predisposing to reactivation.	Evaluate for active disease with CXR and symptom screen; if negative, initiate targeted latent TB preventive therapy.
≥ 15 mm	• Persons with NO known risk factors for tuberculosis	Low pretest probability; high threshold minimizes false-positive diagnoses.	Positive result in general population warrants clinical assessment.

RIPE Antitubercular Pharmacology & Exam Traps

First-line 4-drug therapy for active pulmonary TB (RIPE for 2 months, followed by RI for 4 months).

R — Rifampin

RNA Polymerase Inhibitor

Mechanism: Inhibits bacterial DNA-dependent RNA polymerase, halting transcription.

Adverse Effects: Potent inducer of hepatic cytochrome P450 enzymes (significantly accelerates metabolism of oral contraceptives, warfarin, and protease inhibitors).

Harmless Orange/Red Secretions: Urine, tears, sweat, saliva turn orange; permanently stains soft contact lenses!

I — Isoniazid (INH)

Mycolic Acid Synthesis Inhibitor

Mechanism: Prodrug activated by mycobacterial catalase-peroxidase (KatG); inhibits mycolic acid cell wall synthesis.

Adverse Effects: Dose-dependent peripheral neuropathy due to competition with Pyridoxine (Vitamin B6). Hepatotoxicity (asymptomatic transaminitis or fatal hepatitis).

Always co-administer Pyridoxine (B6)! Triggers Drug-Induced Lupus (anti-histone antibodies in slow acetylators).

P — Pyrazinamide

Acidic Environment Sterilizer

Mechanism: Prodrug converted by pyrazinamidase into pyrazinoic acid; disrupts membrane potential and energy synthesis in acidic phagolysosomes.

Adverse Effects: Hyperuricemia due to competition with uric acid excretion in renal tubules (precipitates acute gouty arthritis). Hepatotoxicity.

Board Pearl: Essential for sterilizing slow-growing intracellular organisms inside macrophage phagolysosomes.

E — Ethambutol

Arabinosyl Transferase Inhibitor

Mechanism: Inhibits arabinosyl transferase, blocking polymerization of arabinogalactan in the mycobacterial cell wall.

Adverse Effects: Retrobulbar optic neuritis presenting with decreased visual acuity, central scotoma, and red-green color blindness.

Mnemonic: "Eye-thambutol causes optic neuritis". Regular visual acuity and Ishihara color testing mandated.

Hansen Disease (Leprosy) & Atypical Nontuberculous Mycobacteria

Immunologic dichotomy of Mycobacterium leprae and high-yield environmental NTM species.

Th1 Response Low Bacillary Load

Tuberculoid Leprosy (Paucibacillary)

Robust cell-mediated Th1 immunity (IL-2, IFN-γ). Macrophages contain the infection. Presents with a few well-demarcated, hypopigmented or erythematous anesthetic skin plaques with raised borders and thickened peripheral nerves.

Lepromin Skin Test: POSITIVE (robust DTH response).
Regimen: Dapsone + Rifampin for 6 months.

Th2 Response High Bacillary Load

Lepromatous Leprosy (Multibacillary)

Defective cell-mediated immunity with blunted Th2 humoral response (IL-4, IL-10). Unchecked bacterial replication in histiocytes. Presents with diffuse skin thickening, multiple nodular lesions, facial skin fold enlargement (leonine facies), loss of eyebrows and eyelashes (madarosis), saddle-nose deformity, and stocking-glove sensory neuropathy.

Lepromin Skin Test: NEGATIVE (anergic).
Regimen: Dapsone + Rifampin + Clofazimine for 12 to 24 months.

Viral Classification & Genomic Taxa

Virology & Genomic Taxa

Comprehensive genomic organization, replication strategies, envelope dynamics, and clinical syndromes of DNA and RNA viruses.

Viral Genomics Enveloped vs. Naked • Herpesviruses • Poxvirus

DNA Viruses Taxonomy & Pathologies

Taxonomy of DNA viruses, envelope origins, linear vs. circular genomes, HPV oncoproteins E6/E7, and sensory ganglia latency sites.

Board Pearl: Poxvirus is the ONLY DNA virus that replicates in host CYTOPLASM (brings its own RNA polymerase)! Parvovirus B19 is the ONLY single-stranded DNA (ssDNA) virus!

Viral Genomics (+) vs. (-) Sense • Segmented BOAR • Enveloped vs. Naked

RNA Viruses Architecture & Clinical Syndromes

(+) vs (-) sense architecture, RNA-dependent RNA polymerase requirement, BOAR segmented genomes, antigenic drift vs. shift, and paramyxovirus fusion protein.

Board Pearl: All (-) sense RNA viruses MUST carry their own RNA-dependent RNA polymerase inside the virion. Mnemonic for segmented viruses: "BOAR" (Bunyavirus, Orthomyxovirus, Arenavirus, Reovirus).

BOAR

Segmented RNA Viruses • Reassortment Risk

All segmented viruses can undergo antigenic shift (genetic reassortment) causing pandemics.

B • Bunyaviridae

3 Segments (Circular)

Hantavirus (pulmonary syndrome, hemorrhagic fever, rodent feces), California encephalitis, Crimean-Congo fever.

O • Orthomyxoviridae

8 Segments (Linear)

Influenza A, B, C. Hemagglutinin (viral entry) and Neuraminidase (viral progeny release). Drift = point mutations (epidemics). Shift = reassortment (pandemics).

A • Arenaviridae

2 Segments (Circular)

Lymphocytic choriomeningitis virus (LCMV), Lassa fever encephalitis. Transmitted by rodents.

R • Reoviridae

10-12 Segments (Double-stranded RNA)

Rotavirus (#1 cause of severe infantile watery diarrhea worldwide; destroys villous enterocytes via NSP4 enterotoxin) and Coltivirus (Colorado tick fever).

DNA Viruses

HHAPPPP Architecture

All DNA viruses are double-stranded EXCEPT Parvovirus (ssDNA). All are linear EXCEPT Papilloma & Polyoma (circular supercoiled) and Hepadna (circular incomplete). All are icosahedral EXCEPT Poxvirus (complex box).

Herpesviridae (Enveloped dsDNA) HSV-1 (oral herpes, temporal lobe hemorrhagic encephalitis with RBCs in CSF, latent in trigeminal ganglia), HSV-2 (genital herpes, aseptic meningitis, latent in sacral ganglia), VZV (varicella/chickenpox, herpes zoster/shingles, post-herpetic neuralgia, latent in dorsal root ganglia), EBV (infects B cells via CD21 / CR2; mononucleosis characterized by heterophile antibodies and reactive atypical lymphocytes that are CD8+ cytotoxic T cells; ampicillin/amoxicillin rash; Burkitt lymphoma t(8;14), nasopharyngeal carcinoma, CNS lymphoma in AIDS), CMV (owl eye intranuclear inclusions, congenital CMV triad of deafness, periventricular calcifications, and microcephaly), HHV-6 (Roseola infantum / exanthem subitum: 3–4 days of high fever followed by maculopapular rash sparing face), HHV-8 (Kaposi sarcoma in AIDS).

Poxviridae (Enveloped Complex dsDNA) Smallpox, Molluscum contagiosum (umbilicated flesh-colored papules with Henderson-Paterson molluscum bodies). Largest virus; carries its own DNA-dependent RNA polymerase and replicates in the cytoplasm!

Parvoviridae (Naked ssDNA) Parvovirus B19: Erythema infectiosum (Fifth disease, "slapped cheek" rash), aplastic crisis in sickle cell disease, hydrops fetalis in utero. Replicates in erythroid precursors.

Structural Virology

Naked vs Enveloped Rule

Enveloped viruses acquire their lipid membrane from host cell membranes (or nuclear membrane for Herpes). Sensitive to ether, acid, and heat → transmitted by droplets/body fluids. Naked (non-enveloped) viruses resist stomach acid/bile → transmitted fecal-oral and survive on fomites!

Naked RNA Viruses (CPR and Hepe)

Caliciviridae: Norovirus (cruise ship viral gastroenteritis).
Picornaviridae: Poliovirus, Echovirus, Coxsackievirus, Rhinovirus (acid-labile, cannot go through GI tract), HAV (fecal-oral hepatitis).
Reoviridae: Rotavirus (infantile diarrhea).
Hepeviridae: Hepatitis E virus (fecal-oral, catastrophic high mortality in pregnant women).

Naked DNA Viruses (PAPP)

Papillomaviridae: HPV 1-4 (warts), HPV 6 & 11 (condylomata acuminata), HPV 16 & 18 (cervical dysplasia/cancer; E6 binds p53, E7 binds Rb).
Adenoviridae: Pharyngoconjunctival fever, acute hemorrhagic cystitis.
Parvoviridae: Parvo B19.
Polyomaviridae: JC virus (PML in HIV), BK virus (polyomavirus-associated nephropathy in renal transplants).

Hepatitis B & C Serologic Interpretation Matrix

Antigen and antibody profiles across acute, window phase, chronic carrier, cleared infection, and vaccination states.

Clinical State	HBsAg	Anti-HBs	HBeAg	Anti-HBe	Anti-HBc IgM	Anti-HBc IgG	Clinical Interpretation & Board Pearls
Acute HBV Infection	(+)	(-)	(+)	(-)	(+)	(-)	Active viral replication. High infectivity (HBeAg+). Anti-HBc IgM is the earliest and most specific antibody marker.
Window Period	(-)	(-)	(-)	(+)	(+)	(-)	Sole diagnostic marker is Anti-HBc IgM! HBsAg has cleared but Anti-HBs has not yet risen to detectable levels.
Chronic HBV (High Infectivity)	(+)	(-)	(+)	(-)	(-)	(+)	HBsAg persistence >6 months defines chronicity. Presence of HBeAg indicates ongoing rapid replication and highest transmission risk.
Chronic HBV (Low Infectivity)	(+)	(-)	(-)	(+)	(-)	(+)	Seroconversion to Anti-HBe indicates lower viral replication, lower infectivity, but risk of reactivation remains.
Recovery / Natural Immunity	(-)	(+)	(-)	(+)	(-)	(+)	Both Anti-HBs and Anti-HBc IgG are POSITIVE! This distinguishes natural resolved infection from immunization.
Vaccinated (Immunized)	(-)	(+)	(-)	(-)	(-)	(-)	ONLY Anti-HBs is POSITIVE! Recombinant HBsAg vaccine produces antibodies to surface antigen only; core antibodies are absent.

HIV Opportunistic Infections Timeline by CD4 Count

Critical diagnostic cutoffs, hallmark pathogens, and evidence-based antimicrobial prophylaxis guidelines.

CD4 < 500 / µL Early Immunodeficiency

Candida albicans: Oral thrush (white plaques easily scraped off leaving erythematous base).
EBV: Oral hairy leukoplakia (white lateral tongue plaques that CANNOT be scraped off).
HHV-8: Kaposi sarcoma (vascular lesions on skin, palate, GI tract).
HPV: Squamous cell carcinoma (anal/cervical intraepithelial neoplasia).
VZV: Severe multidermatomal shingles / herpes zoster.

Prophylaxis: None indicated; initiate standard Antiretroviral Therapy (ART).

CD4 < 200 / µL AIDS-Defining Cutoff

Pneumocystis jirovecii (PJP): Progressive dyspnea on exertion, nonproductive cough, hypoxia, diffuse bilateral interstitial infiltrates, elevated LDH. Silver stain shows cup/crushed ping-pong ball cysts.
JC Virus: Progressive Multifocal Leukoencephalopathy (PML) — non-enhancing multifocal demyelination of subcortical white matter without mass effect.

Primary Prophylaxis: TMP-SMX (Bactrim) 1 DS daily. If sulfa-allergic: Dapsone or Atovaquone. Add Prednisone if PaO2 < 70 or A-a gradient ≥ 35.

CD4 < 100 / µL Severe Vulnerability

Toxoplasma gondii: Multiple ring-enhancing brain lesions on MRI with vasogenic edema and seizures.
Cryptococcus neoformans: Subacute fungal meningoencephalitis; elevated opening pressure, soap-bubble brain lesions, capsular antigen (+) in CSF.
Candida albicans: Esophagitis (substernal burning, odynophagia; white plaques on EGD).
Bartonella henselae: Bacillary angiomatosis (vascular skin lesions, peliosis hepatis).

Primary Prophylaxis: TMP-SMX protects against Toxoplasma if IgG seropositive!

CD4 < 50 / µL Profound Depletion

Mycobacterium avium complex (MAC): Disseminated fever, night sweats, weight loss, diarrhea, hepatosplenomegaly, elevated alkaline phosphatase.
Cytomegalovirus (CMV): Retinitis (pizza-pie fundus, yellow-white retinal infiltrates with hemorrhage → blindness), linear deep esophageal ulcers, colitis with bloody diarrhea.
EBV: Primary CNS Lymphoma (single ring-enhancing periventricular brain lesion; distinction from multiple in Toxo).

Management: Immediate ART. MAC prophylaxis with Azithromycin was standard if not on ART. CMV retinitis treated with Ganciclovir or Valganciclovir.

Clinical Virology Master Matrix (Sketchy Taxonomy)

Complete genomic classifications, hallmark receptors, clinical syndromes, and board-favorite diagnostic associations.

Virus Family & Genome	Key Pathogens & Receptors	Pathognomonic Manifestations & Sketchy Clues	Management & Board Pearls
Picornaviridae (+)ssRNA • Naked • Icosahedral	Poliovirus, Coxsackievirus A & B, Echovirus, Rhinovirus, HAV Receptors: ICAM-1 (Rhinovirus), CD155 (Polio)	• Coxsackie A: Hand-foot-mouth disease (vesicles on palms, soles, oral mucosa), herpangina (vesicular pharyngitis). • Coxsackie B: Dilated cardiomyopathy, acute pericarditis, Bornholm disease (pleurodynia / "Devil's grip" intercostal pain). • Polio: Destroys anterior horn motor neurons → asymmetric flaccid paralysis. • Rhinovirus: Acid-labile (destroyed by stomach acid → upper respiratory tract only, optimal growth at 33°C).	Poliovirus vaccines: Salk (inactivated / killed injectable, IgG only) vs Sabin (live attenuated oral, mucosal IgA + IgG; risk of reversion to virulence). Supportive care.
Flaviviridae & Togaviridae (+)ssRNA • Enveloped	HCV, Dengue, Yellow Fever, West Nile, Zika (Flavi); Rubella, Chikungunya (Toga) Vector: Aedes aegypti & Culex mosquitoes	• Dengue: "Breakbone fever" (severe retro-orbital headache, back/bone pain), thrombocytopenia, shock on secondary infection. • West Nile: Neuroinvasive disease in elderly → asymmetric flaccid paralysis mimicking polio + encephalitis. • Rubella (German Measles): Congenital triad: cataracts, patent ductus arteriosus (PDA), sensorineural deafness + "blueberry muffin" rash. Cephalocaudal rash with postauricular lymphadenopathy in adults.	Live attenuated MMR vaccine prevents Rubella (contraindicated in pregnancy / CD4 < 200). HCV treated with direct-acting antivirals (Sofosbuvir + Ledipasvir / Velpatasvir).
Paramyxoviridae (-)ssRNA • Enveloped • Non-segmented	Measles (Rubeola), Mumps, RSV, Parainfluenza (Croup) Receptors: CD46 / CD150 (Measles), Sialic acid (Mumps/Parainfluenza)	• Measles: 4 Cs: Cough, Coryza, Conjunctivitis, Copious Koplik spots (pathognomonic white spots on buccal mucosa). Maculopapular rash begins at hairline. Long-term complication: Subacute Sclerosing Panencephalitis (SSPE). Warthin-Finkeldey syncytial multinucleated giant cells. • Mumps: Parotitis, orchitis (testicular inflammation → risk of post-pubertal sterility), aseptic meningitis. • RSV: #1 cause of bronchiolitis in infants <1 yr. Fusion (F) protein mediates syncytia. • Parainfluenza: Croup (acute laryngotracheobronchitis) → "seal-bark" cough, inspiratory stridor, "steeple sign" on neck X-ray.	Vitamin A supplementation reduces measles morbidity and mortality. Palivizumab (mAb against F protein) provides RSV prophylaxis in premature infants. Croup: cool mist, dexamethasone, racemic epinephrine.
Rhabdoviridae & Filoviridae (-)ssRNA • Bullet-shaped / Filamentous	Rabies virus, Ebola virus, Marburg virus Rabies Receptor: Nicotinic Acetylcholine Receptor (AChR) at motor endplate	• Rabies: Binds AChR → ascends retrograde along axons via dynein to dorsal root ganglia & brain → hydrophobia (painful pharyngeal spasms upon swallowing water), agitation, autonomic instability, hypersalivation, coma, death. Histology: eosinophilic cytoplasmic Negri bodies in Purkinje cells of cerebellum and pyramidal neurons of hippocampus. • Ebola: Direct contact with body fluids / infected bats → petechiae, purpura, massive DIC, hypovolemic shock.	Rabies post-exposure prophylaxis (mandatory after bat/raccoon bite): Rabies vaccine (killed) + Human Rabies Immune Globulin (HRIG) infiltrated into wound and IM. Once symptoms appear, almost 100% fatal!
Parvoviridae, Adeno, Pox & Polyoma DNA Viruses • Specialized Pathologies	Parvovirus B19 (ssDNA), Adenovirus (dsDNA), Polyomaviruses JC/BK, Molluscum (Pox)	• Parvovirus B19: Binds P antigen on erythroid precursors. Erythema infectiosum (Fifth disease: bright red "slapped cheek" rash + lacy reticular rash on body). Triggers aplastic crisis in sickle cell/spherocytosis. In utero infection → severe fetal anemia, congestive heart failure, and hydrops fetalis. • JC Virus: Progressive Multifocal Leukoencephalopathy (PML) in AIDS/natalizumab → demyelination of oligodendrocytes. • BK Virus: BK nephropathy and hemorrhagic cystitis in renal transplant allografts. • Molluscum Contagiosum: Flesh-colored, firm, dome-shaped umbilicated papules with central cheesy core (Henderson-Paterson intracytoplasmic bodies).	Adenovirus vaccine used in military recruits. Parvo B19 aplastic crisis requires supportive transfusion. Molluscum is self-limited or treated with cryotherapy/curettage.

Medical Mycology & Tropical Parasitology

Fungi, Protozoa & Helminths

Diagnostic morphology, geographic distributions, life cycles, and clinical pharmacology for medical mycoses and parasitic infections.

Medical Mycology Cold=Mold, Heat=Yeast • Endemics • Molds & Branching

Systemic Dimorphic & Opportunistic Mycoses

Geographic dimorphic fungi (Histoplasma, Blastomyces, Coccidioides, Paracoccidioides), opportunistic yeasts (Candida, Cryptococcus, PJP), and mold branching angles.

Board Pearl: Candida albicans is a reverse dimorphic fungus: forms pseudohyphae and budding yeasts at 20°C, but forms TRUE germ tubes at 37°C body temperature!

Tropical Medicine Intestinal • Blood/Tissue • Nematodes • Trematodes • Cestodes

Medical Parasitology: Protozoa & Helminths

Protozoan morphology, falling leaf motility, Maltese crosses, ring-enhancing lesions, Scotch tape pinworm test, and fluke egg spines.

Board Pearl: Schistosoma haematobium eggs possess a large TERMINAL spine and predispose specifically to SQUAMOUS CELL CARCINOMA of the urinary bladder!

Systemic Dimorphic Fungi

"Cold = Mold (20°C); Warm = Yeast (37°C)". Endemic granulomatous pulmonary infections.

Histoplasma capsulatum

Ohio & Mississippi Valleys

Bird & bat droppings (spelunking/caves, chicken coops).

Histology: Tiny oval intracellular yeast bodies packed inside macrophages (smaller than RBC).

Clinical: Palatal/tongue ulcers, hepatosplenomegaly, pancytopenia in immunocompromised.

Mnemonic: "Histo Hides in Histiocytes (macrophages)".

Blastomyces dermatitidis

Eastern & Central US / Great Lakes

Rotting wood, organic matter, soil.

Histology: Large, round yeast with broad-based budding and doubly refractile thick wall (same size as RBC).

Clinical: Verrucous/ulcerative skin lesions, osteomyelitis (lytic bone lesions), pulmonary disease.

Mnemonic: "Broad-based Budding = Blastomyces".

Coccidioides immitis

Southwestern US / San Joaquin Valley

Desert dust storms, earthquake tremors.

Histology: Huge thick-walled spherules packed with endospores (much larger than RBC).

Clinical: San Joaquin Valley fever: erythema nodosum (desert bumps), arthralgias (desert rheumatism), meningitis.

Mnemonic: "Coccidioides Crowded with Endospores in Spherule".

Paracoccidioides brasiliensis

Latin & South America

Rural agricultural workers, coffee plantation soil.

Histology: Multiple daughter buds radiating around a parent yeast cell → classic "Captain's wheel" or "Mariner's wheel" appearance.

Clinical: Mucocutaneous painful ulcerations of mouth and nose, cervical lymphadenopathy.

Mnemonic: "Captain's Wheel drives to Brazil".

High-Yield Clinical Parasites

Classic buzzwords, transmission routes, and Board-favorite diagnostic findings.

Protozoa • GI Tract

Giardia & Entamoeba

Giardia lamblia: Campers/hikers drinking unpurified stream water. Bloating, flatulence, foul-smelling greasy steatorrhea. Trophozoite: teardrop-shaped with 2 nuclei ("owl eyes"). Treat with Metronidazole.

Entamoeba histolytica: Dysentery (bloody diarrhea), flask-shaped intestinal ulcers, solitary right hepatic abscess ("anchovy paste" exudate). Trophozoites engulf RBCs (erythrophagocytosis).

Protozoa • CNS & Systemic

Toxoplasma & Malaria

Toxoplasma gondii: Cat feces / undercooked pork. In HIV (CD4 < 100): multiple ring-enhancing brain lesions. Congenital triad: chorioretinitis, hydrocephalus, diffuse intracranial calcifications. Prophylaxis: TMP-SMX.

Plasmodium (Malaria): Anopheles mosquito. Giemsa blood smear: trophozoite ring form and Schüffner dots. P. vivax & ovale have dormant liver hypnozoites → MUST add Primaquine (check G6PD first!).

Nematodes • Helminths

Enterobius & Ascaris

Enterobius vermicularis (Pinworm): Nocturnal perianal pruritus in children. Female worm migrates out of anus to lay eggs. Diagnosis: Cellophane "Scotch tape" test. Treat child AND entire household with Pyrantel pamoate or Albendazole.

Ascaris lumbricoides: Giant roundworm. Fecal-oral ingestion of eggs → larvae penetrate gut → lung migration (Loeffler eosinophilic pneumonitis) → swallowed → adult worms cause intestinal obstruction or biliary colic.

Opportunistic Fungi & Cutaneous Mycoses

Angioinvasive molds, yeast pseudohyphae, dimorphic rose thorn inoculations, and keratinophilic dermatophytes.

Zygomycetes Wide 90° Non-Septate

Mucor & Rhizopus

Broad, ribbon-like, non-septate hyphae branching at wide right (90°) angles.

Vulnerability: Ketoacidotic diabetics (DKA - ketone reductase utilizes host ketones) and neutropenic leukemia patients.

Pathology: Rhinocerebral mucormycosis: spores proliferate in blood vessel walls (angioinvasive), penetrate cribriform plate → cavernous sinus thrombosis, cranial nerve palsies, and black necrotic eschar on palate or nasal mucosa.

Emergency surgical debridement + IV Amphotericin B / Isavuconazole.

Thermal Dimorphic Cigar-Shaped Yeast

Sporothrix schenckii

"Rose gardener's disease". Mold on vegetation/soil; cigar-shaped budding yeast at 37°C.

Clinical: Traumatic inoculation by rose thorn or sphagnum moss → painless subcutaneous nodule that ulcerates, followed by ascending nodular lymphangitis along lymphatic draining channels.

Histology: Cigar-shaped budding yeasts; asteroid bodies in granulomas.

Rx: Oral Itraconazole or saturated solution of potassium iodide (SSKI).

Superficial & Cutaneous KOH Mount

Malassezia & Dermatophytes

Lipid-dependent normal flora vs Keratin-consuming molds.

Malassezia furfur (Pityriasis versicolor): Degradation of lipids yields azelaic acid, which damages melanocytes → hypo- or hyperpigmented macules on chest/back. KOH shows "spaghetti and meatballs" (short hyphae + yeast cells). Rx: Topical Selenium sulfide.

Dermatophytes (Microsporum, Trichophyton, Epidermophyton): Branching septate hyphae on KOH. Cause Tinea pedis (athlete's foot), cruris (jock itch), corporis (ringworm), capitis, and unguium (onychomycosis). Rx: Terbinafine or Azoles.

Master Parasitology & Helminthology Matrix

Protozoa, Nematodes (roundworms), Cestodes (tapeworms), and Trematodes (flukes) with transmission routes, life cycles, and drug regimens.

Organism & Class	Vector / Route & Cycle	Clinical Manifestation & Sketchy Associations	Diagnostics & Treatment
Trypanosoma cruzi Flagellated Kinetoplastid	Reduviid bug ("kissing bug") feces deposited during painless nocturnal bite near lips/eyes	Chagas Disease: Acute: painless unilateral periorbital edema (Romaña sign). Chronic: destroys intramural autonomic myenteric (Auerbach) plexus → Dilated cardiomyopathy with apical aneurysm (leading cause of HF in Latin America), Megaesophagus (dysphagia), and Megacolon (severe constipation).	Blood smear for trypomastigotes (acute); PCR/serology. Rx: Benznidazole or Nifurtimox.
Trypanosoma brucei gambiense / rhodesiense	Tsetse fly (Glossina) bite with painful chancre	African Sleeping Sickness: Cyclical recurring fevers (due to variable surface glycoprotein [VSG] antigenic variation), posterior cervical lymphadenopathy (Winterbottom sign), followed by CNS invasion: progressive daytime somnolence, behavioral changes, ataxia, coma, and death.	Trypomastigotes in blood smear or CSF. Rx: Suramin for early hemolymphatic disease; Melarsoprol for CNS disease ("It SURe is nice to sleep; MELAtonin for sleep").
Cryptosporidium parvum Apicomplexan Protozoan	Fecal-oral ingestion of chlorine-resistant oocysts in water (public pools, municipal water supply)	Immunocompetent: mild, watery, self-limited diarrhea. AIDS patients (CD4 < 100): severe, intractable, voluminous watery diarrhea, dehydration, malabsorption, and electrolyte wasting.	Stool examination: Acid-fast round oocysts (4-5 µm) staining bright magenta. Rx: ART restoration; Nitazoxanide in immunocompetent.
Strongyloides stercoralis Intestinal Nematode	Filariform larvae in soil penetrate intact skin of bare feet → lungs → swallow → adults in intestine	Autoinfection cycle: Rhabditiform larvae in gut lumen mature into infectious filariform larvae, penetrating perianal skin/gut wall → persistent infection for decades. In immunocompromised / steroid therapy: hyperinfection syndrome with septic shock and Gram-negative polymicrobial meningitis.	Stool microscopy shows rhabditiform larvae (NOT eggs!). Eosinophilia. Rx: Ivermectin (or Albendazole).
Trichinella spiralis Tissue Nematode	Eating undercooked meat containing encysted larvae (domestic pork or wild game / bear meat)	Larvae mature in gut → invade bloodstream → encyst inside striated muscle cells. Triad of periorbital edema, severe debilitating myositis (muscle aches), and subungual splinter hemorrhages. Marked eosinophilia, elevated CK and LDH.	Muscle biopsy reveals nurse cells containing coiled encysted larvae. Rx: Albendazole or Mebendazole + Corticosteroids for severe inflammation.
Ancylostoma & Necator Hookworms (Nematodes)	Filariform larvae penetrate bare feet ("ground itch") → lungs → swallowed → intestinal villi	Adult worms attach to intestinal mucosa and consume host blood (up to 0.2 mL/worm/day). Leading cause of severe microcytic hypochromic iron deficiency anemia and lethargy in developing nations.	Stool examination shows oval eggs with clear halos. Rx: Albendazole or Pyrantel pamoate + oral iron supplementation.
Diphyllobothrium latum Fish Tapeworm (Cestode)	Ingestion of larvae in raw or undercooked freshwater fish (sushi, sashimi, pike)	Longest human tapeworm (up to 10-15 meters!). Competes with host enterocytes for dietary Vitamin B12 → severe Megaloblastic anemia with hypersegmented neutrophils, elevated methylmalonic acid/homocysteine, and subacute combined degeneration of spinal cord (ataxia, paresthesias).	Proglottids and operculated eggs in stool. Rx: Praziquantel + Vitamin B12 replacement.
Echinococcus granulosus Dog Tapeworm (Cestode)	Ingestion of eggs from dog feces; sheep are intermediate hosts (sheep herders in rural areas)	Hydatid Cyst Disease: Slow-growing, slow-expansile multilocular cystic liver lesion with characteristic "eggshell" calcification on CT scan. CRITICAL BOARD PEARL: Accidental cyst rupture or aspiration spillage of "hydatid sand" fluid can precipitate catastrophic fatal anaphylactic shock!	Abdominal CT/ultrasound; serology. Rx: PAIR procedure (Puncture, Aspirate, Inject hypertonic saline/alcohol scolicide, Re-aspirate) or surgical excision under cover of Albendazole.
Clonorchis sinensis Chinese Liver Fluke (Trematode)	Ingestion of encysted metacercariae in undercooked freshwater fish; snails are 1st host	Adult flukes dwell in human biliary tree → chronic biliary inflammation, recurrent cholangitis, pigmented brown gallstones, and a 15-fold increased risk of Cholangiocarcinoma (biliary duct adenocarcinoma).	Small operculated eggs with shoulders in stool. Rx: Praziquantel.

Clinical Decision Matrix CSF Differential • Sepsis-3 • SBP Paracentesis • Endocarditis

CSF Diagnostics & Critical ID Syndromes

Diagnostic matrix distinguishing bacterial vs. viral vs. fungal/TB CSF parameters, Sepsis-3 SOFA thresholds, SBP paracentesis criteria, and Duke endocarditis criteria.

Board Pearl: In bacterial meningitis, CSF glucose is markedly DECREASED (< 40 mg/dL or CSF:serum < 0.4) with PMN predominance; in viral meningitis, CSF glucose is completely NORMAL with lymphocyte predominance.

Syndromic Infectious Disease

Clinical Syndromes & CSF Interpretation Matrix

Quantitative CSF profiles across meningitis etiologies, infective endocarditis stratifications, osteomyelitis triggers, and diarrheal syndromes.

Meningitis Staged by Age Group & CSF Diagnostic Table

Laboratory parameters, opening pressures, glucose ratios, and empiric antimicrobial regimens.

Etiology	Opening Pressure	WBC Count & Differential	Glucose (CSF:Serum)	Protein Concentration	Primary Pathogens & Board Pearls
Normal Baseline	50–200 mm H2O	<5 cells / µL (all lymphs)	>0.6 (approx. 45–80 mg/dL)	15–45 mg/dL	Acellular, clear, colorless fluid.
Acute Bacterial	Markedly Elevated (>250 mm H2O)	>1,000–10,000 / µL (>80% PMNs / Neutrophils)	Markedly Low (<0.4 / <40 mg/dL)	Markedly High (>100–500 mg/dL)	Turbid/purulent. Neonates (<1 mo): GBS, E. coli K1, Listeria → Ampicillin + Cefotaxime/Gentamicin. Children/Adults: S. pneumoniae, N. meningitidis → Vancomycin + Ceftriaxone + Dexamethasone. Adults >50 / Immunocompromised: Add Ampicillin to cover Listeria!
Viral (Aseptic)	Normal to Mildly ↑	10–500 / µL (predominantly Lymphocytes)	Normal (>0.6)	Normal to Mildly ↑ (<100 mg/dL)	Clear fluid. Enteroviruses (Coxsackievirus, Echovirus), HSV-2 (Mollaret meningitis), Arboviruses, HIV. Normal glucose is the key differentiator! Supportive care.
Fungal (Cryptococcal)	Markedly Elevated (>250 mm H2O)	20–500 / µL (predominantly Lymphocytes)	Low (<0.4)	Elevated (>100–500 mg/dL)	Cryptococcus neoformans in HIV (CD4 < 100). High opening pressure requires serial therapeutic LPs. India ink / cryptococcal antigen positive. Induction: Amphotericin B + Flucytosine.
Tuberculous (TB)	Elevated (>250 mm H2O)	100–500 / µL (Lymphocytic predominance)	Extremely Low (<0.3 / <30 mg/dL)	Extremely High (>100–1000 mg/dL)	Mycobacterium tuberculosis. Basilar meningeal enhancement, cranial nerve palsies (CN III, VI, VII). CSF forms "pellicle / spiderweb" clot on standing. Rx: RIPE + Dexamethasone.

Cardiovascular Infections

Infective Endocarditis Stratification

Acute Native Valve (S. aureus) High fever, rapid destruction of normal valves. Tricuspid valve involvement in IV drug users (holosystolic murmur ↑ with inspiration [Carvallo sign]; septic pulmonary emboli).

Subacute Native Valve (Strep viridans) S. mutans, S. sanguinis synthesize dextrans adhering to fibrin-platelet nidus on previously damaged valves (mitral prolapse, bicuspid aortic) after dental procedures.

Prosthetic Valve Endocarditis • Early (<60 days): Staph epidermidis (biofilm former; treat with Vancomycin + Gentamicin + Rifampin).
• Late (>60 days): Strep viridans or S. aureus.

Colon Cancer Clue & HACEK Group • Streptococcus gallolyticus (bovis): Mandates immediate colonoscopy to rule out occult colorectal adenocarcinoma!
• Culture-Negative HACEK: Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella + Coxiella burnetii (Q fever) and Bartonella.

MSK & Bone Infections

Osteomyelitis & Septic Arthritis

Sickle Cell Disease: Salmonella > S. aureus Vaso-occlusive bone micro-infarctions and hyposplenism predispose to Salmonella enterica osteomyelitis (H2S-positive motile rod). Treat with Ceftriaxone or Ciprofloxacin.

Animal Bites & Sneaker Puncture • Cat or Dog Bite: Pasteurella multocida causes fulminant cellulitis and osteomyelitis within <24 hours. Rx: Augmentin.
• Puncture through Rubber Sneaker Sole: Pseudomonas aeruginosa colonizes rubber; causes calcaneal osteochondritis.

Sexually Active Young Adult Monoarthritis Neisseria gonorrhoeae causes asymmetric migratory tenosynovitis, purulent monoarthritis (knee), and pustular dermatitis on extremities. Synovial fluid Gram stain often negative; test urine/cervix NAAT.

GI Pathophysiology

Diarrheal Syndromes Dichotomy

Inflammatory / Bloody (Dysentery) Mucosal invasion and cytotoxins → fecal leukocytes (fecal lactoferrin/calprotectin +), fever, tenesmus:

Campylobacter jejuni (poultry, 42°C, GBS).
EHEC O157:H7 (Shiga-like toxin, HUS; DO NOT give antibiotics!).
Shigella dysenteriae (actin rockets, low infectious dose).
Salmonella enterica (poultry, eggs, turtles).
Yersinia enterocolitica (pork, pseudoappendicitis).
Entamoeba histolytica (flask ulcers, liver abscess).

Secretory / Watery (Non-Inflammatory) Fluid hypersecretion without mucosal destruction (fecal WBCs negative):

Vibrio cholerae & ETEC (cAMP / cGMP).
Rotavirus (NSP4 enterotoxin in infants) & Norovirus (cruises).
Giardia & Cryptosporidium (protozoa).
Preformed Toxins: S. aureus (mayo/custard <6h), B. cereus (fried rice <5h).

Critical Care Infectious Disease

Sepsis Bundle, Septic Shock & Neutropenic Fever

Sepsis & Septic Shock 1-Hour Bundle

qSOFA Score (≥2 high risk): RR ≥22/min, Altered mentation (GCS <15), SBP ≤100 mmHg.
Lactate & Blood Cultures: Serum lactate level + 2 sets of blood cultures drawn BEFORE initiating antimicrobials.
Resuscitation: Rapid IV infusion of 30 mL/kg balanced crystalloids for hypotension (MAP <65 mmHg) or lactate ≥4 mmol/L.
Vasopressors: Norepinephrine is the first-line vasopressor for septic shock refractory to fluid loading (target MAP ≥65 mmHg).

Neutropenic Fever Protocol

Single oral temperature ≥38.3°C (101.0°F) or sustained ≥38.0°C (100.4°F) for >1 hour with Absolute Neutrophil Count (ANC) <500 / µL (or expected nadir <500).

Immediate empiric monotherapy with anti-pseudomonal β-lactam: Cefepime, Meropenem, or Piperacillin-Tazobactam within 60 minutes! (Add Vancomycin only if hemodynamic instability, catheter infection, pneumonia, or severe mucositis).

Surgical & Abdominal Emergencies

Necrotizing Soft Tissue & Peritoneal Sepsis

Necrotizing Fasciitis (Type I vs Type II)

Type I (Polymicrobial): Enteric Gram-negative rods + anaerobes in diabetics; includes Fournier gangrene of perineum/scrotum.
Type II (Monomicrobial): Streptococcus pyogenes (Group A Strep) ± S. aureus; rapid progression.
Hallmark: Severe pain disproportionate to physical findings, skin bullae, crepitus, rapid edema.
Mandatory Step: Immediate emergent surgical exploration and wide operative debridement! Do NOT delay surgery for imaging!
Antimicrobial Triad: IV Vancomycin + Piperacillin-Tazobactam + Clindamycin (clindamycin halts bacterial protein/toxin synthesis at 50S ribosome).

Spontaneous Bacterial Peritonitis (SBP)

Infection of ascitic fluid without an intra-abdominal surgically treatable source in cirrhotic patients with ascites. Pathogens: E. coli, Klebsiella, Streptococcus pneumoniae.

• Diagnostic Paracentesis: Ascitic fluid PMNs ≥ 250 / µL.
• Regimen: IV Cefotaxime or Ceftriaxone + IV Albumin (1.5 g/kg day 1, 1.0 g/kg day 3) to prevent hepatorenal syndrome.
• Secondary Prophylaxis: Daily oral Fluoroquinolone or TMP-SMX for life!

Pulmonary Medicine

Pneumonia Syndromes & CURB-65 Admission Triage

CURB-65 Risk Stratification Confusion, Urea >20 mg/dL (BUN >7 mmol/L), Respiratory rate ≥30/min, BP <90 systolic or ≤60 diastolic, Age ≥65.

Score 0–1 = Outpatient management. Score 2 = Inpatient floor admission. Score ≥3 = Consider ICU admission.

Community-Acquired Pneumonia (CAP) • Outpatient healthy: Amoxicillin OR Doxycycline.
• Outpatient with comorbidities: Amoxicillin-Clavulanate + Azithromycin, OR Respiratory Fluoroquinolone (Levofloxacin/Moxifloxacin).
• Inpatient non-ICU: Ceftriaxone + Azithromycin, OR Levofloxacin monotherapy.

HAP, VAP & Aspiration Pneumonia • HAP / VAP (≥48h in hospital): Dual antipseudomonal + MRSA coverage (Vancomycin + Cefepime / Zosyn).
• Aspiration Pneumonia: Right lower lobe / superior segment in supine patients; oral anaerobes (Peptostreptococcus, Fusobacterium, Prevotella); treat with Ampicillin-Sulbactam or Clindamycin.

GU & Sexually Transmitted Infections

UTIs, Asymptomatic Bacteriuria & Genital Ulcers

UTI & Asymptomatic Bacteriuria Rules

Uncomplicated Cystitis: Nitrofurantoin (5 days), TMP-SMX (3 days), or Fosfomycin (single dose). Avoid fluoroquinolones!
Pyelonephritis / Complicated UTI: Ciprofloxacin, Levofloxacin, or Ceftriaxone.
Asymptomatic Bacteriuria: Screen and treat ONLY in: 1) Pregnant women (risk of pyelonephritis, preterm labor; treat with Cephalexin, Nitrofurantoin [avoid 1st trimester/term], or Augmentin), and 2) Prior to invasive urologic procedures with mucosal bleeding. DO NOT treat in catheterized patients or elderly!

Pelvic Inflammatory Disease (PID) Cervical motion tenderness ("chandelier sign"), adnexal tenderness, uterine tenderness. Complications: Tubo-ovarian abscess, Fitz-Hugh-Curtis syndrome, ectopic pregnancy, infertility.

Outpatient: IM Ceftriaxone (500 mg) + oral Doxycycline (100 mg BID x 14d) + oral Metronidazole (500 mg BID x 14d). Inpatient: IV Cefoxitin/Cefotetan + IV Doxycycline.

Resistance Genetics mecA / PBP2a • vanA D-Ala-D-Lac • ESBL • CRE • Efflux

Antimicrobial Resistance & "Superbugs"

Molecular genetics and biochemistry of antimicrobial resistance: MRSA, VRE terminal dipeptide substitution, ESBL, CRE metallo-beta-lactamases, and Pseudomonas multidrug barriers.

Board Pearl: mecA encodes PBP2a with low affinity for beta-lactams (MRSA). vanA substitutes D-Ala-D-Ala with D-Ala-D-Lac (VRE). Ceftaroline is the only beta-lactam that binds PBP2a.

Pharmacology & Resistance Genetics

Antimicrobial Resistance & Superbug Strategy

Genetic mechanisms of multidrug resistance, high-yield molecular alterations, and evidence-based drug-of-choice regimens.

Altered Target PBP

MRSA: mecA & PBP2a

Mechanism: Acquired mobile cassette (SCCmec) carrying the mecA gene, which encodes an altered penicillin-binding protein, PBP2a.

Clinical Consequence: PBP2a has markedly reduced binding affinity for virtually all β-lactam antibiotics, conferring complete cross-resistance to all penicillins, cephalosporins (1st–4th gen), and carbapenems!

Only β-lactam active against MRSA: Ceftaroline (5th generation cephalosporin).

Altered Peptidoglycan Precursor

VRE: vanA & D-Ala-D-Lac

Mechanism: Transposon Tn1546 carries the vanA operon, which substitutes the terminal amino acid of cell wall peptidoglycan pentapeptide from D-Ala-D-Ala to D-Ala-D-Lac (D-lactate).

Clinical Consequence: Vancomycin requires terminal D-Ala-D-Ala for hydrogen bonding. D-Ala-D-Lac causes a 1,000-fold loss of vancomycin binding affinity → high-level vancomycin resistance in E. faecium & E. faecalis.

Drugs of Choice: Linezolid or Daptomycin.

Enzymatic Hydrolysis

ESBL, AmpC & CRE

ESBL (TEM, SHV, CTX-M): Plasmid-mediated β-lactamases in E. coli and Klebsiella that hydrolyze penicillins, cephalosporins (1st–4th gen), and aztreonam. Drug of Choice: Carbapenems (Meropenem).

AmpC β-Lactamase: Inducible chromosomal cephalosporinase in Enterobacter, Serratia, Citrobacter. Cephalosporin exposure selects for derepressed mutants. Treat with Cefepime or Carbapenems.

CRE (KPC, NDM-1 metallo-β-lactamase): Hydrolyzes carbapenems. Treat with Ceftazidime-avibactam, Meropenem-vaborbactam, or Colistin.

Superbug Drug-of-Choice & High-Yield Board Traps

First-line options for resistant pathogens and critical clinical pharmacology contraindications.

Resistant Pathogen	First-Line Antimicrobial Regimen	Alternative Regimens	Critical Board Traps & Adverse Effects
MRSA (Bacteremia / Skin)	Vancomycin or Daptomycin	Linezolid, Ceftaroline, Doxycycline (oral mild)	DAPTOMYCIN SURFACTANT INACTIVATION: Daptomycin binds pulmonary surfactant and is completely inactivated! NEVER use Daptomycin for MRSA Pneumonia! Use Vancomycin or Linezolid for MRSA pneumonia.
VRE (Vancomycin-Resistant Enterococcus)	Linezolid or Daptomycin	Tigecycline, Oritavancin	Linezolid: Causes bone marrow suppression (thrombocytopenia after >2 weeks), optic/peripheral neuropathy, and Serotonin Syndrome when combined with SSRIs due to weak nonselective MAO inhibition!
Pseudomonas aeruginosa	Cefepime or Piperacillin-Tazobactam (Zosyn)	Meropenem, Ceftazidime, Ciprofloxacin, Aztreonam	In severe penicillin/cephalosporin anaphylaxis, use Aztreonam (monobactam, zero cross-reactivity with penicillins EXCEPT ceftazidime). Resistance via OprD porin loss & MexAB efflux pumps.
ESBL Enterobacterales	Carbapenems (Meropenem, Imipenem)	Ceftolozane-tazobactam, Eravacycline	Imipenem is administered with Cilastatin (inhibits renal dehydropeptidase I, preventing renal inactivation). High doses in renal impairment can trigger seizures!
Anaerobes (Bacteroides, Clostridium)	Metronidazole or Ampicillin-Sulbactam	Clindamycin (above diaphragm), Carbapenems	Metronidazole generates cytotoxic free radicals in anaerobic conditions. Adverse: disulfiram-like reaction with alcohol; metallic taste. Clindamycin has high risk of inducing C. diff colitis.

Public Health & Safety Airborne vs. Droplet vs. Contact • Needlestick PEP • Rabies/Tetanus

Transmission Precautions & Post-Exposure Prophylaxis

Hospital transmission precautions (Airborne "Air MTV", Droplet, Contact), occupational needlestick transmission risks (HBV 30%, HCV 3%, HIV 0.3%), and urgent PEP regimens.

Board Pearl: Alcohol-based hand rubs DO NOT eradicate Clostridioides difficile spores! Healthcare personnel must wash hands with antimicrobial soap and warm running water!

Hospital Epidemiology & Prevention

Infection Control, Precautions & Vaccinology Suite

Transmission-based precautions (Contact, Droplet, Airborne), post-exposure prophylaxis protocols, and vaccine immunology.

Hospital Transmission-Based Precautions Matrix

CDC isolation criteria, required personal protective equipment (PPE), and target infectious pathogens.

Precaution Tier	Required PPE & Room Requirements	Target Pathogens & Clinical Syndromes	High-Yield Board Pearl
Standard Precautions	Hand hygiene (alcohol-based rub or soap/water); gloves, gown, or mask only when anticipation of contact with blood/body fluids.	Universal to ALL hospitalized patients regardless of infection status.	Hand hygiene before and after every patient contact is the #1 intervention to reduce nosocomial infections.
Contact Precautions	Gown and gloves donned before room entry. Dedicated single-patient equipment (stethoscope, BP cuff). Private room.	MRSA, VRE, scabies, draining abscesses, RSV (infants), multi-drug resistant Gram-negatives (ESBL, CRE).	Always remove gown and gloves and perform hand hygiene BEFORE exiting the patient's room.
Special Contact (C. diff)	Gown and gloves. MUST WASH HANDS WITH SOAP AND WATER (alcohol rubs do NOT kill bacterial spores!). Bleach wipes.	Clostridioides difficile colitis, Norovirus outbreaks.	Alcohol-based hand sanitizer is completely INEFFECTIVE against C. diff endospores! Mechanical hand washing with soap and water is mandatory.
Droplet Precautions	Surgical face mask donned upon room entry; eye protection (goggles/face shield) if within 3 to 6 feet of patient.	Neisseria meningitidis, Haemophilus influenzae type b (epiglottitis/meningitis), Influenza virus, Bordetella pertussis, Mumps, Rubella, Adenovirus.	Droplets are large particles (> 5 µm) that travel ≤ 3–6 feet and do not remain suspended in the air.
Airborne Precautions	Fit-tested N95 respirator or PAPR before entry. Airborne Infection Isolation Room (AIIR) with negative pressure and ≥ 12 air exchanges/hour, exhausted outdoors.	Measles (Rubeola), Tuberculosis (active pulmonary/laryngeal), Varicella-Zoster (chickenpox, disseminated shingles).	Mnemonic: "Air MTV" = Measles, Tuberculosis, Varicella. Droplet nuclei (< 5 µm) stay suspended indefinitely.

Post-Exposure Prophylaxis (PEP) Pathways

Immediate time-sensitive clinical algorithms for occupational needlesticks, animal bites, and close contacts.

Needlestick Emergency

HIV Occupational Exposure

Transmission risk from hollow-bore percutaneous stick is ~0.3%. Initiate PEP immediately within 72 hours (ideally < 2 hours!).

3-Drug Regimen x 28 days: Tenofovir + Emtricitabine + Dolutegravir (or Raltegravir).

Hepatitis B Needlestick

HBV Vaccination & Titer Check

Check worker's anti-HBs titer. If anti-HBs ≥ 10 mIU/mL, the healthcare worker is fully immune → No PEP needed!

Unvaccinated or Non-responder: Administer Hepatitis B Immune Globulin (HBIG) + start HBV vaccine series within 24 hours.

Bat or Carnivore Bite

Rabies Post-Exposure

Immediate vigorous wound irrigation with soap and water. Waking up with a bat in the room mandates PEP even without visible marks!

Unvaccinated: Rabies Immune Globulin (RIG) infiltrated into/around wound + Rabies vaccine on days 0, 3, 7, 14.

Close Contact Prophylaxis

Neisseria meningitidis

Close household contacts, dormitory roommates, daycare classmates, or healthcare worker directly exposed to oral secretions (intubation).

First-line options: Ciprofloxacin 500 mg PO single dose, Rifampin 600 mg PO BID x 2 days, or Ceftriaxone 250 mg IM (preferred in pregnancy).

Tick Attachment

Borrelia burgdorferi

Prophylaxis criteria: 1) Adult/nymph Ixodes scapularis, 2) Attached ≥ 36 hours and engorged, 3) Endemic area, 4) Initiated ≤ 72 hours of removal.

Drug of Choice: Single-dose oral Doxycycline 200 mg. If criteria not met: active observation for erythema migrans.

Varicella Exposure

VZV Prophylaxis

Non-immune healthy exposed: Administer Varicella vaccine within 5 days of exposure.

Immunocompromised, pregnant, or newborn of mother with varicella: Administer Varicella-Zoster Immune Globulin (VariZIG) within 10 days!

Osteopathic Principles & Practice

DO / COMAT & Level-Specific Microbiology Correlates

Viscerosomatic autonomic innervation levels, Chapman's reflex diagnostic points, and OMT safety rules in infectious diseases.

Viscerosomatic Autonomic Reflex Innervation Table

Spinal cord segments mediating tissue texture changes, palpatory hypertonicity, and autonomic hypersympathetonia.

Anatomical System	Sympathetic Levels	Parasympathetic Origin	Collateral Ganglion	Infectious Clinical Correlate
Head & Neck / ENT	T1–T4	Vagus (CN X) / CN VII, IX	Superior Cervical Ganglion	Acute bacterial otitis media, bacterial sinusitis, streptococcal pharyngitis.
Lower Respiratory / Lungs	T2–T7	Vagus (CN X)	Pulmonary Plexus	Lobar pneumonia (S. pneumoniae), acute bronchitis, aspiration pneumonitis. Somatic dysfunction at T2–T7.
Upper GI (Stomach, Liver, Gallbladder)	T5–T9	Vagus (CN X)	Celiac Ganglion	H. pylori peptic ulcer disease, acute cholecystitis, hepatitis, amebic liver abscess.
Middle GI (Small Bowel, Appendix, Asc. Colon)	T9–T11	Vagus (CN X)	Superior Mesenteric (SMG)	Acute appendicitis, infectious enteritis (Salmonella, Shigella, Giardia), SBP in cirrhotics.
Lower GI (Desc. Colon, Sigmoid, Rectum)	T12–L2	Pelvic Splanchnics (S2–S4)	Inferior Mesenteric (IMG)	C. diff pseudomembranous colitis, diverticulitis, infectious proctitis (Chlamydia, Gonorrhea).
Kidneys & Upper Ureters	T10–T11	Vagus (CN X)	Aorticorenal Ganglion	Acute pyelonephritis (E. coli, Proteus, Klebsiella), renal abscess.
Lower Ureters & Bladder	T11–L2	Pelvic Splanchnics (S2–S4)	Inferior Hypogastric Plexus	Acute bacterial cystitis (E. coli, S. saprophyticus, Enterococcus).

High-Yield Chapman's Diagnostic Reflex Points

Neurolymphatic gangliform contractions palpable as pea-sized, exquisitely tender nodules.

Sinuses & Pharynx

Sinusitis & Pharyngitis

Sinuses: Anterior: 2nd intercostal space near sternum. Posterior: Midway between spinous and transverse process of T2.

Pharynx: Anterior: 1st intercostal space near sternum. Posterior: Midway between spinous and transverse process of T2.

Bronchi & Lungs

Pneumonia & Bronchitis

Bronchi: Anterior: 2nd intercostal space near sternum. Posterior: Between T2 and T3 transverse processes.

Upper Lung: Anterior: 3rd intercostal space. Posterior: Between T3 and T4 transverse processes.

Lower Lung: Anterior: 4th intercostal space. Posterior: Between T4 and T5 transverse processes.

Appendix

Acute Appendicitis

Anterior Point: Tip of the 12th rib on the right anteriorly.

Posterior Point: Transverse process of T11 vertebra on the right posteriorly.

Classic board question: Palpable nodule at the right 12th rib tip = Appendix!

OMT Safety & Absolute Contraindications in Infectious Pathology

When to treat, when to modify, and when OMT is strictly forbidden.

Absolute Contraindications

Do NOT Perform OMT In These Conditions:

Active Bacteremia & Septic Shock: Vigorous lymphatic pump techniques (thoracic pump, pedal pump) risk dislodging bacterial emboli and exacerbating systemic hemodynamic collapse.
Acute Bacterial Osteomyelitis: HVLA, articulatory techniques, or local deep myofascial release over the infected bone are strictly contraindicated due to risk of cortical fracture and septic hematoma.
Acute Septic Arthritis: No direct joint mobilization or HVLA on an actively infected synovial joint.
Acute Surgical Abdomen / Peritonitis: Ruptured appendix, hollow viscus perforation, toxic megacolon; mesenteric release is strictly contraindicated.

Safe & Indicated Adjuncts

Evidence-Based OMT in Resolving Infection:

Rib Raising & Suboccipital Release: Normalizes excessive sympathetic hyperactivity (T2–T7 for pneumonia, T5–T9 for GI), reduces bronchial secretions, and improves chest wall compliance.
Thoracic Inlet Myofascial Release: Releases scalenes, clavicle, and 1st rib to open terminal drainage of the thoracic duct into the left subclavian vein.
Dome of the Diaphragm Release: Restores full excursion of the thoracoabdominal diaphragm to optimize pressure gradients driving lymphatic return in recovering pneumonia patients.

Immunology in ID Asplenia • CGD (Catalase +) • C5-C9 Defect • HIV CD4 Strata

Host Susceptibility & Immunodeficiency States

Inborn and acquired host susceptibility states: encapsulated sepsis in asplenia, catalase-positive vulnerability in CGD, recurrent Neisseria in MAC defects, and opportunistic AIDS infections by CD4 count.

Board Pearl: Chronic Granulomatous Disease (CGD) involves an NADPH oxidase defect resulting in recurrent infections with CATALASE-POSITIVE organisms ("PLACES": Pseudomonas, Listeria, Aspergillus, Candida, E. coli, S. aureus, Serratia, Nocardia).

Immunology • Host Defense

Primary Immunodeficiencies & Microbial Predispositions

Genetic immune lesions and their pathognomonic opportunistic pathogen pairings for COMLEX and USMLE.

Immunodeficiency Disorder	Genetic Defect & Mechanism	Susceptible Pathogens & Board Clues	Diagnostic Confirmation & Clinical Pearls
Chronic Granulomatous Disease (CGD) X-linked recessive (gp91phox)	Defect in NADPH Oxidase → inability of neutrophils to generate respiratory burst (superoxide O2- and H2O2).	Recurrent infections with Catalase-Positive Organisms (catalase destroys host H2O2, depriving neutrophil of myeloperoxidase substrate): "Cats Need PLACESS to Belch": • Candida, Nocardia, Pseudomonas, Listeria, Aspergillus, Corynebacterium, E. coli, Staph aureus, Serratia, Burkholderia cepacia.	Dihydrorhodamine (DHR) flow cytometry (decreased green fluorescence) or Nitroblue Tetrazolium (NBT) dye reduction test (fails to turn blue). Prophylaxis: TMP-SMX + Itraconazole + IFN-γ.
Terminal Complement Deficiency C5, C6, C7, C8, C9 Deficiencies	Inability to assemble the Membrane Attack Complex (MAC / C5b-9) → defective lysis of thin-walled Gram-negative bacteria.	Recurrent, severe, invasive Neisseria meningitidis (meningococcemia, purpura fulminans) and Neisseria gonorrhoeae infections in teens/young adults.	Screen with total hemolytic complement assay (CH50: <10% of normal). Meningococcal vaccination and prophylactic antibiotics.
Asplenia (Sickle Cell / Post-Splenectomy) Loss of splenic splenic macrophages & tuftsin	Loss of splenic marginal zone B-cells, reduced IgM, and absence of mechanical filtration of poorly opsonized organisms.	Fulminant, life-threatening septic shock from Encapsulated Bacteria: • Streptococcus pneumoniae (#1 killer). • Haemophilus influenzae type b. • Neisseria meningitidis. • Salmonella enterica osteomyelitis. • Babesia microti (splenic filtration of intraerythrocytic parasites lost → severe hemolytic crisis!).	Peripheral blood smear: Howell-Jolly bodies (nuclear DNA remnants), target cells, thrombocytosis. Mandatory immunization: Pneumococcal (PCV13 + PPSV23), Meningococcal, Hib. Empiric Ceftriaxone for any fever!
Hyper-IgE (Job Syndrome) Autosomal dominant STAT3 mutation	Impaired Th17 differentiation → failure of neutrophils to recruit to cutaneous infection sites.	Recurrent "Cold" non-inflamed Staphylococcal abscesses (lack warmth and erythema!), severe atopic eczema, chronic mucocutaneous candidiasis.	FATED Mnemonic: Coarse Facies, cold staph Abscesses, retained primary Teeth, elevated IgE (>2,000 IU/mL) + eosinophilia, Dermatologic eczema / bone fractures.
Wiskott-Aldrich Syndrome X-linked recessive WAS gene defect	Defective actin cytoskeleton reorganization in leukocytes and platelets → impaired antigen presentation and platelet shedding.	Recurrent pyogenic sinopulmonary infections (encapsulated bacteria) and opportunistic viral/fungal infections.	WATER Mnemonic: Wiskott-Aldrich, Thrombocytopenia with microthrombocytes (small platelets; distinguishes from ITP!), Eczema, Recurrent pyogenic infections. Low IgM, elevated IgA/IgE.
X-Linked Agammaglobulinemia (Bruton) BTK gene defect on X chromosome	Defective Bruton tyrosine kinase (BTK) → pro-B cells cannot mature into pre-B cells. Total absence of mature CD19/CD20 B cells and plasma cells.	Presents at 6 months of age (after maternal transplacental IgG wanes) with recurrent pyogenic respiratory infections (S. pneumo, H. flu), severe Enterovirus (polio, coxsackie CNS infections), and chronic Giardia lamblia diarrhea (lack of mucosal IgA).	Flow cytometry shows absent circulating CD19/CD20 B-cells; profoundly low IgG, IgA, IgM. Absent tonsils and germinal centers on lymph node biopsy! Regular IVIG replacement.