Medical Microbiology (Bacteriology, Virology, Mycology, Parasitology)
Medical Microbiology tests specimen selection, staining, culture, identification, susceptibility testing, and direct detection across bacteria, viruses, fungi, and parasites. The exam emphasizes interpretable results, not memorized organism lists.
Core rules
- Gram stain guides early therapy: Gram-positive cocci in clusters suggest staphylococci; in chains suggest streptococci. Catalase and coagulase help identify Staphylococcus aureus.
- Culture media are selective or differential; MacConkey agar selects Gram-negative bacilli and differentiates lactose fermenters. MALDI-TOF is common for colony identification.
- Susceptibility testing uses MIC or disk diffusion with standardized inoculum and incubation. ESBL and carbapenemase phenotypes require specific confirmation.
- Virology relies on molecular NAAT, antigen tests, and serology. IgM suggests recent infection but can cross-react; IgG indicates exposure or immunity.
- Mycology: KOH mounts show fungal elements; culture and morphology identify molds and yeasts. Parasitology uses stool O&P, malaria thick/thin films, and antigen tests.
Worked example
A wound swab grows Gram-positive cocci in clusters. The isolate is catalase positive and coagulase positive. Interpretation: Staphylococcus aureus. Reasoning: clusters and catalase positivity fit staphylococci; coagulase positivity separates S. aureus from coagulase-negative staphylococci. The next step is susceptibility testing, not automatic reporting as contamination.
Traps
Normal flora and colonizers can obscure pathogens. Sputum with many squamous cells is a poor lower-respiratory specimen. A single negative culture does not exclude infection. Do not perform susceptibility testing on contaminants without clinical context. Handle AFB and fungal cultures under appropriate biosafety conditions.