Microbiology accounts for roughly 10–15% of Step 1 and is one of the most memorization-dense sections on the exam. The good news: it's highly learnable with the right strategy.
The Right Way to Learn Microbiology for Step 1
Rote memorization of bug lists fails. The students who master microbiology learn it through association — connecting each organism to its virulence factors, clinical presentation, transmission route, and treatment. Sketchy Microbiology uses visual mnemonics to encode these associations into long-term memory and is the highest-yield microbiology resource available.
For each organism, know: Gram stain result, shape, key virulence factors, associated disease, classic patient presentation, and first-line treatment. Build this framework in Anki using the AnKing deck's Sketchy Micro tags.
Gram-Positive Bacteria: High-Yield Organisms
Gram-positive cocci
| Organism | Key features | Classic diseases | Treatment |
|---|---|---|---|
| Staph aureus | Coagulase positive, protein A (anti-phagocytic), TSST-1 | Skin infections, osteomyelitis, endocarditis, food poisoning (preformed toxin), TSS | Nafcillin (MSSA); vancomycin (MRSA) |
| Staph epidermidis | Coagulase negative, biofilm formation | Prosthetic valve endocarditis, IV line infections | Vancomycin |
| Strep pyogenes (GAS) | Beta-hemolytic, M protein, streptolysin O (ASO titer) | Pharyngitis, scarlet fever, rheumatic fever, impetigo, necrotizing fasciitis | Penicillin G |
| Strep pneumoniae | Alpha-hemolytic, lancet-shaped diplococci, polysaccharide capsule | Pneumonia (most common), otitis media (#1 in kids), meningitis, sinusitis | Penicillin G (if sensitive) |
| Enterococcus | Gamma-hemolytic, bile esculin positive, NaCl tolerant | UTI, endocarditis (especially in elderly) | Ampicillin; vancomycin-resistant: linezolid/daptomycin |
Gram-positive rods
| Organism | Key clue | Disease |
|---|---|---|
| Bacillus anthracis | Spore-forming, anthrax toxin, "medusa head" colonies | Cutaneous (eschar), pulmonary ("woolsorter's disease"), GI anthrax |
| Bacillus cereus | Fried rice poisoning (preformed toxin) | Vomiting (1–6h) or diarrhea (8–16h) |
| Clostridium perfringens | Spore-forming, gas gangrene, lecithinase (alpha toxin) | Gas gangrene, food poisoning |
| Clostridium difficile | Pseudomembranous colitis, toxin A (enterotoxin) + toxin B (cytotoxin) | Antibiotic-associated colitis; treat with fidaxomicin or vancomycin PO |
| Listeria monocytogenes | Tumbling motility, cold enrichment, meningitis in immunocompromised/neonates | Neonatal meningitis, food-borne illness in pregnancy; treat with ampicillin |
| Corynebacterium diphtheriae | Club-shaped, metachromatic granules, ADP-ribosylation of EF-2 | Pseudomembranous pharyngitis, myocarditis, neuropathy; treat with antitoxin + erythromycin |
Gram-Negative Bacteria: High-Yield Organisms
| Organism | Key virulence | Classic presentation | Transmission |
|---|---|---|---|
| Neisseria meningitidis | Polysaccharide capsule, IgA protease, endotoxin (LPS) | Bacterial meningitis + petechial rash, Waterhouse-Friderichsen syndrome | Respiratory droplets |
| Neisseria gonorrhoeae | Pili, IgA protease, no capsule (variable) | Urethritis, PID, septic arthritis (most common in young adults), neonatal conjunctivitis | Sexual contact |
| Haemophilus influenzae | Type b: polysaccharide capsule (PRP); IgA protease | Meningitis (post-vaccine), epiglottitis (thumb sign on X-ray), otitis media | Respiratory droplets; treat with ceftriaxone |
| Pseudomonas aeruginosa | Exotoxin A (EF-2 inhibitor), blue-green pigment (pyocyanin) | Burn wound infections, pneumonia (CF patients), hot tub folliculitis, malignant otitis externa | Water, hospital environment |
| Klebsiella pneumoniae | Mucoid capsule, "currant jelly" sputum | Lobar pneumonia (alcoholics, diabetics), UTI, liver abscess | Endogenous flora |
| E. coli | ETEC (heat-labile/stable toxins), EHEC O157:H7 (Shiga-like toxin → HUS) | UTI (#1 cause), traveler's diarrhea (ETEC), HUS (EHEC), neonatal meningitis | Fecal-oral, contaminated food |
| Salmonella typhi | Intracellular, Vi antigen | Typhoid fever: rose spots, relative bradycardia, constipation then diarrhea | Fecal-oral; fomite — gallbladder carrier |
| Helicobacter pylori | Urease (alkaline environment), VacA, CagA | Peptic ulcer disease (duodenal > gastric), gastric adenocarcinoma, MALT lymphoma | Fecal-oral; treat with triple therapy |
Atypical Bacteria (No Cell Wall)
| Organism | Key features | Disease + Treatment |
|---|---|---|
| Mycoplasma pneumoniae | No cell wall → beta-lactams ineffective; cold agglutinins | "Walking pneumonia" — young adults, mild illness, X-ray worse than patient looks; treat azithromycin |
| Chlamydia trachomatis | Intracellular; elementary body (infectious) → reticulate body (replicating) | D–K: STI (urethritis, PID, neonatal conjunctivitis/pneumonia); A–C: trachoma; L1–L3: LGV |
| Rickettsia rickettsii | Obligate intracellular; Weil-Felix reaction; tick-borne | Rocky Mountain spotted fever: fever, rash on palms/soles spreading centrally; treat doxycycline |
| Treponema pallidum | Too thin to Gram stain; darkfield microscopy | Syphilis: 1° chancre → 2° rash/condylomata → latent → 3° gummas/tabes dorsalis/aortitis |
High-Yield Viruses
DNA viruses mnemonic: HHAPPPy
Herpesviruses, Hepadnavirus (HBV), Adenovirus, Parvovirus, Papillomavirus, Poxvirus, Polyomavirus — all DNA viruses.
| Virus | Classic association |
|---|---|
| HSV-1 / HSV-2 | Oral cold sores (1), genital herpes (2), encephalitis (temporal lobe), neonatal herpes |
| VZV | Chickenpox (primary); shingles (reactivation in dorsal root ganglion) |
| EBV | Mono (heterophile antibodies, atypical lymphocytes), Burkitt's lymphoma, nasopharyngeal carcinoma, PTLD |
| CMV | Mono (heterophile negative), retinitis in AIDS, "owl-eye" inclusion bodies, congenital CMV |
| HPV 6/11 | Genital warts (condylomata acuminata) |
| HPV 16/18 | Cervical cancer, oropharyngeal cancer (E6 degrades p53, E7 degrades Rb) |
| Parvovirus B19 | Fifth disease (slapped cheek), aplastic crisis in sickle cell, hydrops fetalis |
Fungi: The Most Tested
| Fungus | Key association | Treatment |
|---|---|---|
| Candida albicans | Oral thrush (immunocompromised), esophagitis (AIDS), vulvovaginitis, diaper rash; pseudohyphae | Fluconazole (systemic); nystatin (topical) |
| Aspergillus fumigatus | Septate hyphae at 45° angles; invasive aspergillosis in neutropenic patients; allergic bronchopulmonary aspergillosis | Voriconazole |
| Cryptococcus neoformans | India ink stain; soap-bubble lesions in basal ganglia; meningitis in AIDS (CD4 <100) | Amphotericin B + flucytosine, then fluconazole |
| Histoplasma capsulatum | Ohio/Mississippi river valleys, bat/bird droppings; intracellular macrophages; calcified granulomas | Itraconazole (mild), amphotericin B (severe) |
| Coccidioides immitis | Southwest USA/California (desert); spherules with endospores; Valley Fever | Fluconazole (mild), amphotericin B (severe) |
| Pneumocystis jirovecii | PCP pneumonia in AIDS (CD4 <200); bilateral interstitial infiltrates; GMS stain; prophylaxis TMP-SMX | TMP-SMX; add steroids if PaO2 <70 |
Sketchy strategy: For every Sketchy Micro video, watch it once to absorb the story, then immediately add the corresponding AnKing Anki cards. Come back to the video one more time the following day before moving on to the next organism. Three exposures — video, Anki cards, video — locks it in better than anything else.
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