Medical Microbiology and Infectious Disease Pathology
Medical Microbiology and Infectious Disease Pathology tests how organisms are sampled, stained, cultured and identified, and how host response produces tissue patterns. The exam expects bacterial, mycobacterial, fungal, viral and parasitic morphology plus basic antimicrobial reasoning.
Core rules
- Gram stain separates bacteria by cell-wall structure; culture remains essential for identification and susceptibility.
- Mycobacteria are acid-fast; caseating granulomas suggest tuberculosis but are not specific.
- Fungi are classified by morphology: yeasts, hyphae, pseudohyphae and dimorphism. Special stains include PAS and silver stains.
- Viral inclusions and cytopathic effects can suggest a family but usually require PCR or serology.
- Sepsis is a clinical syndrome; blood cultures before antibiotics improve yield.
- Not every positive culture is infection; contamination and colonization must be distinguished.
Worked example
A 40-year-old man has cough, weight loss and night sweats. Lung biopsy shows caseating granulomas with Langhans giant cells, and Ziehl-Neelsen stain shows acid-fast bacilli. The leading diagnosis is tuberculosis. Reasoning: caseation plus acid-fast bacilli in a compatible host is strong evidence. Culture and molecular testing confirm and assess resistance. Nontuberculous mycobacteria can produce similar granulomas, so identification matters.
Traps
Do not diagnose tuberculosis from a granuloma alone. A positive blood culture may be a contaminant, especially a single coagulase-negative staphylococcus bottle. Candida in respiratory secretions often reflects colonization. Clostridioides difficile disease requires toxin or molecular testing plus clinical diarrhea, not stool culture alone.