Microbiology: Sterilisation, Culture and Serology
Microbiology in NEET PG rewards precise laboratory detail: which sterilisation method suits which material, which medium selects which organism, and which serological pattern defines the stage of infection. Bacteriology, virology, mycology and parasitology all appear, but hepatitis B serology, culture media and toxin mechanisms recur most often.
Core rules
- Autoclave: 121 degrees Celsius at 15 psi for 15 minutes. Hot-air oven: 160 degrees Celsius for 2 hours, for glassware and sharp instruments. Ethylene oxide for heat-labile plastics and endoscopes.
- MacConkey agar: lactose fermenters such as E. coli form pink colonies; non-fermenters such as Shigella and Salmonella remain pale.
- Chocolate agar supplies both X and V factors for Haemophilus influenzae; satelliitism around Staphylococcus aureus is a classic clue.
- Weil-Felix is a nonspecific agglutination test and cannot confirm rickettsial species.
Worked example
A blood sample is HBsAg negative, anti-HBc IgM positive and anti-HBs negative. This is the window period: surface antigen has cleared but protective antibody has not yet appeared, so the only detectable marker is core IgM. HBV DNA is the marker of infectivity here. Exposed contacts need hepatitis B immunoglobulin plus vaccine. Compare this with anti-HBs alone, which indicates vaccination or past resolved infection, and with HBsAg plus HBeAg, which indicates high infectivity.
Traps
- Reading anti-HBs positivity as active infection rather than immunity.
- Forgetting that cold agglutinins and a positive Mycoplasma PCR, not culture, confirm atypical pneumonia.
- Confusing the CAMP test (group B Streptococcus) with the reverse CAMP reaction of Listeria.