Microbiology
What is tested
Microbiology questions cluster around organism identification from a short vignette, culture media, toxin mechanisms, hepatitis and HIV serology, and sterilization parameters.
Core rules
- Gram stain logic: Gram-positive cocci in clusters = staphylococci (catalase positive; S. aureus coagulase positive). Chains = streptococci (S. pyogenes bacitracin sensitive; S. agalactiae hippurate positive; S. pneumoniae optochin sensitive).
- Media: MacConkey for lactose fermenters (pink colonies); TCBS for Vibrio cholerae; Lowenstein–Jensen for mycobacteria; chocolate agar for Haemophilus and Neisseria.
- Toxins: diphtheria and Pseudomonas exotoxin A block EF-2; cholera toxin and E. coli heat-labile toxin raise cAMP; tetanus and botulinum block neurotransmitter release; endotoxin is lipid A of lipopolysaccharide.
- Hepatitis B serology: HBsAg appears first; anti-HBc IgM marks acute infection and the window period; anti-HBs indicates immunity.
- Sterilization: autoclave 121 °C at 15 psi for 15 minutes; hot-air oven 160 °C for 2 hours.
Worked example
A blood donor is HBsAg negative, anti-HBs positive and anti-HBc IgG positive. Reasoning: anti-HBs alone would suggest vaccination, but anti-HBc IgG is not produced by vaccination - it requires natural infection. The pattern therefore indicates resolved infection with residual immunity, and the donor is not infectious.
Traps
The window period (anti-HBc IgM positive, HBsAg and anti-HBs negative) is misread as susceptibility; HBeAg is confused with HBsAg; and vaccination is wrongly credited with producing anti-HBc.