Microbiology — Buzzwords — USMLE Step 2 CK Notes

Toxin mechanisms

Encapsulated organisms (asplenia risk)

Mnemonic — Some Killers Have Nice Shiny Bodies: Streptococcus pneumoniae, Klebsiella, Haemophilus influenzae, Neisseria, Salmonella, group B Strep. These require opsonisation, so asplenic patients need vaccination and prompt treatment.

Classic stains and media

• India ink / capsule → Cryptococcus
• Silver stain → Pneumocystis, fungi, H. pylori
• Chocolate agar with factors V and X → Haemophilus
• Charcoal yeast extract with cysteine and iron → Legionella

Gram-positive organisms

• Staph aureus — catalase+, coagulase+; abscesses, endocarditis (IVDU, tricuspid), toxic shock (TSST-1 superantigen), scalded skin, food poisoning (preformed toxin, fast onset).
• Strep pyogenes (Group A) — pharyngitis, impetigo, cellulitis; sequelae rheumatic fever and post-strep glomerulonephritis; scarlet fever, necrotising fasciitis.
• Strep pneumoniae — lancet-shaped diplococci; #1 community-acquired pneumonia, otitis media, meningitis; encapsulated (asplenia risk).
• Strep agalactiae (Group B) — neonatal meningitis/sepsis; screen mothers at 35-37 weeks.
• Enterococcus — UTIs, biliary; vancomycin resistance (VRE).
• Clostridia — C. tetani (spastic paralysis, blocks glycine/GABA), C. botulinum (flaccid, blocks ACh release), C. perfringens (gas gangrene), C. difficile (pseudomembranous colitis).
• Corynebacterium diphtheriae — pseudomembrane, myocarditis; toxin blocks EF-2. Listeria — unpasteurised dairy, pregnancy, neonates; tumbling motility.

Gram-negative organisms

• Neisseria — meningitidis (meningococcaemia, waterhouse-friderichsen) and gonorrhoeae (no vaccine, no maltose fermentation).
• E. coli — UTI, neonatal meningitis (K1), watery (ETEC) or bloody (EHEC O157:H7 → HUS, don't give antibiotics) diarrhoea.
• Pseudomonas — blue-green pigment, fruity odour; burns, cystic fibrosis, hospital-acquired; ecthyma gangrenosum.
• Klebsiella — currant-jelly sputum, aspiration/alcoholics. H. influenzae — epiglottitis, meningitis (type b). Legionella — atypical pneumonia + hyponatraemia + diarrhoea (water sources).
• Helicobacter pylori — ulcers, gastric cancer, MALT lymphoma; urease+.
• Campylobacter/Salmonella/Shigella/Vibrio — inflammatory/bloody diarrhoea; Campylobacter → Guillain-Barré.

Atypicals, spirochetes & mycobacteria

• Mycoplasma — walking pneumonia, cold agglutinins; no cell wall (no beta-lactam).
• Chlamydia — obligate intracellular; urethritis/PID (D-K), trachoma/LGV. Rickettsia — Rocky Mountain spotted fever (wrists/ankles inward).
• Spirochetes — Treponema pallidum (syphilis stages), Borrelia (Lyme — erythema migrans), Leptospira.
• Mycobacterium tuberculosis — acid-fast, caseating granuloma, Ghon complex; TB reactivation with immunosuppression.

Viruses, fungi & parasites (high-yield)

• DNA viruses — Herpes (HSV, VZV, CMV owl-eye, EBV → mono/lymphoma), HBV, HPV (koilocytes, E6/E7 → cervical cancer).
• RNA viruses — influenza (antigenic shift/drift), HIV (retrovirus, CD4 decline), hepatitis A/C/E, rabies (Negri bodies).
• Fungi — Candida (thrush, immunocompromised), Aspergillus (aspergilloma, invasive), Cryptococcus (meningitis, India ink), Pneumocystis (PCP at CD4<200), dimorphics (Histoplasma, Blastomyces, Coccidioides).
• Parasites — Plasmodium (malaria), Entamoeba (dysentery, liver abscess), Giardia (fatty stools), Toxoplasma (ring lesions), Trypanosoma.