Media, Reagents, and Stains
Core concepts
Know general-purpose media (blood agar, chocolate agar, MacConkey, TSA), selective media (MacConkey for Gram-negatives, mannitol salt for staphylococci, CNA for Gram-positives, HE/KIA for enteric), differential media (blood agar for hemolysis, MacConkey for lactose fermentation, KIA for carbohydrate fermentation and H2S), and enriched media (chocolate agar for fastidious organisms). Understand broth media (thioglycollate for anaerobes, LIM broth for GBS).
Know stains: Gram stain (crystal violet, iodine, decolorizer, safranin), acid-fast (Ziehl-Neelsen, Kinyoun, fluorochrome auramine-rhodamine), fungal stains (calcofluor white, lactophenol cotton blue, PAS, GMS), and special stains (Giemsa, Wright, India ink for Cryptococcus capsule, silver stain).
Traps to expect
- Confusing selective with differential - many media are both (MacConkey is selective and differential).
- Forgetting that chocolate agar is enriched, not selective, and supports Haemophilus and Neisseria.
- Overlooking that Gram stain decolorization is the critical, timing-sensitive step.
- Assuming India ink identifies the organism - it demonstrates the capsule, not the species.
Worked example / rule of thumb
A CSF specimen from an immunocompromised patient shows encapsulated yeast on India ink. Rule of thumb: India ink demonstrates the capsule of Cryptococcus neoformans; confirm with India ink plus cryptococcal antigen (CrAg). The capsule is the diagnostic clue, not the ink itself.