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.

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.

M-Protein: Antiphagocytic virulence factor; molecular mimicry triggers Rheumatic Fever (Jones criteria: Joints, Carditis, Nodules, Erythema marginatum, Sydenham chorea).

Sequelae: Rheumatic fever occurs ONLY after pharyngitis. Post-streptococcal glomerulonephritis (PSGN) can occur after pharyngitis OR skin impetigo!

Early penicillin therapy prevents Rheumatic Heart Disease, but does NOT prevent PSGN!

Catalase (-) • β-Hemolytic Bacitracin RESISTANT

Streptococcus agalactiae (Group B)

CAMP test positive (arrowhead hemolysis with S. aureus). Hippurate test positive.

Clinical Focus: #1 cause of neonatal sepsis, neonatal meningitis, and neonatal pneumonia.

Screening & Prophylaxis: Pregnant women screened at 35-37 weeks via rectovaginal swab. Intrapartum IV Penicillin G or Ampicillin given if positive.

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

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 & sanguinis: Normal oral flora. S. sanguinis makes dextrans from glucose to adhere to pre-damaged heart valves (subacute bacterial endocarditis after dental procedures).

Enterococcus (faecalis / faecium): Grows in 6.5% NaCl and bile (PYR+). Causes biliary tract infections, subacute endocarditis after GI/GU procedures. VRE encodes vanA gene (D-Ala-D-Lac substitution).

Strep gallolyticus (bovis): Grows in bile but NOT 6.5% NaCl. Associated with colon cancer when presenting as bacteremia/endocarditis!

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.

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).
EHEC (O157:H7): Shiga-like toxin, 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) Curved rod colonizing gastric antrum. Strong Urease producer (converts urea to ammonia → alkaline microenvironment protects against stomach acid). Causes chronic antral gastritis, duodenal and gastric peptic ulcers, gastric adenocarcinoma, and MALT lymphoma. Quadruple therapy: Bismuth + 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.

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

Mycoplasma pneumoniae

Structure: Completely lacks a peptidoglycan cell wall (naturally resistant to all beta-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: 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.

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.

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.

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, temporal lobe encephalitis with RBCs in CSF), HSV-2 (genital, neonatal), VZV (chickenpox, shingles), EBV (infectious mononucleosis, heterophile antibody +, Burkitt lymphoma, nasopharyngeal carcinoma), CMV (owl eye inclusions, congenital deaf/calcifications), HHV-6 (Roseola / exanthem subitum: high fever 3-4 days then rash), HHV-8 (Kaposi sarcoma).

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.

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.