Medical Microbiology and Infectious Disease Pathology
Overview
This domain tests the bridge between the microbiology laboratory and tissue diagnosis: which organism morphology belongs to which class, how susceptibility is reported, and how infection control and biosafety rules apply.
Core rules
- Gram stain and culture remain the reference standard for most bacterial infections; MALDI-TOF identifies colonies rapidly, and molecular panels shorten turnaround for fastidious organisms.
- Susceptibility: MIC values are interpreted against CLSI or EUCAST breakpoints; the choice of standard must be consistent within a laboratory.
- Hyphae on H&E: Aspergillus shows narrow, septate, acute-angle branching hyphae with invasion of vessels; Mucorales show broad, ribbon-like, non-septate hyphae with right-angle branching.
- Viral inclusions: CMV produces large owl-eye intranuclear inclusions with cytoplasmic inclusions; HPV produces koilocytic change with perinuclear halos.
- Granulomas: caseating granulomas with AFB positivity indicate tuberculosis; non-caseating granulomas suggest sarcoidosis or other causes.
- Biosafety: mycobacterial culture and prion work require appropriate containment levels, and all clinical specimens are handled with standard precautions.
Worked example
A diabetic patient with rhino-orbital necrosis has tissue showing broad, ribbon-like, aseptate hyphae branching at right angles with extensive angioinvasion. Reasoning: the morphology identifies a Mucorales organism, not Aspergillus; the clinical setting fits mucormycosis. Urgent debridement plus high-dose amphotericin B is indicated. Calling this Aspergillus would delay correct therapy.
Traps
Confusing colonization with infection when interpreting a respiratory culture. Overlooking that a positive blood culture can represent a skin contaminant such as coagulase-negative staphylococcus. Ignoring the difference between intrinsic and acquired resistance.