Microbiology
Microbiology
This domain tests specimen adequacy, stain interpretation, organism identification, and antimicrobial susceptibility reporting - the path from Gram stain to a clinically useful result.
Core rules
- Gram stain reporting includes shape, arrangement, and Gram reaction; neutrophils with intracellular organisms in a normally sterile site are significant.
- Catalase and coagulase distinguish staphylococci: S. aureus is coagulase positive; coagulase-negative staphylococci are not. Streptococci are catalase negative and classified by hemolysis (beta, alpha, gamma).
- MIC is the lowest concentration inhibiting visible growth; it is interpreted against CLSI or EUCAST breakpoints as susceptible, intermediate, or resistant. MBC is the lowest concentration killing 99.9% of the inoculum.
- Blood culture yield depends on volume (about 20–30 mL per adult set) and collection before antibiotics; most bacteremia is detected within 24–48 hours.
- MALDI-TOF identifies colonies from protein spectra but does not replace susceptibility testing.
Worked example
A wound culture grows Gram-positive cocci in clusters; colonies are catalase positive, coagulase positive, and ferment mannitol. Identification: Staphylococcus aureus. If the oxacillin/cefoxitin MIC is 8 µg/mL, the isolate is methicillin resistant (MRSA) and all beta-lactams except those with documented activity (e.g., ceftaroline) are reported resistant. Reporting only the MIC without applying breakpoints is incomplete.
Traps that cost marks
- Performing susceptibility testing on a poor-quality or mixed culture.
- Reporting a single positive blood culture bottle without assessing contamination risk (e.g., coagulase-negative staph).
- Confusing MIC with MBC.
- Forgetting that inducible resistance (D-test) matters for erythromycin/clindamycin reporting.