Microbiology
Core concepts
Microbiology questions blend organism identification, specimen selection, and antimicrobial logic. Know the Gram stain and colony morphology of major pathogens, the meaning of catalase and coagulase tests, and the biochemical panels used for Enterobacterales. Understand culture media (MacConkey, chocolate, Thayer-Martin, CIN) and when each is indicated. Master anaerobic specimen handling (avoiding air exposure, rejecting superficial swabs) and the significance of sterile-site versus colonizing-site cultures. Know antimicrobial susceptibility testing concepts: MIC, breakpoints, disk diffusion, and resistance mechanisms (ESBL, MRSA via mecA, vancomycin resistance via vanA/vanB). Recognize reportable public health organisms and biosafety levels.
Traps
A common trap is interpreting a positive culture from a nonsterile site as infection when it may reflect colonization. Another is misidentifying contaminants (coagulase-negative staphylococci in a single blood culture bottle). Questions may ask which specimen is unacceptable (sputum with >25 squamous cells per low-power field, superficial wound swabs for anaerobes). Distinguish intrinsic from acquired resistance and remember that inducible resistance (D-test for clindamycin) matters for staphylococci. Do not assume a Gram stain result equals final identification.
Worked example / rule of thumb
For a suspected anaerobic infection, collect aspirated fluid or tissue, not a swab, and transport in an anaerobic system. Rule of thumb: if the specimen cannot be obtained without contamination from a colonized surface, the culture result is hard to interpret-choose the most sterile accessible site.