Microbiology
Core concepts
Microbiology questions reward a specimen-to-answer workflow: proper collection and transport, direct Gram stain, culture on selective and differential media, colony and biochemical identification, then susceptibility testing. Know what an MIC and a breakpoint mean, how disk diffusion works, and the resistant phenotypes requiring specific reporting: MRSA (mecA, cefoxitin screen), ESBLs, carbapenemases, inducible clindamycin resistance (D-test), and vancomycin-resistant enterococci. Blood cultures should be drawn before antibiotics, in two sets from separate sites, with adequate volume driving yield.
Traps
- Distinguish contamination (coagulase-negative staphylococci, diphtheroids, viridans streptococci in a single bottle) from true bacteremia.
- Normal flora interpretation is site-dependent; a sputum Gram stain dominated by squamous epithelial cells indicates salivary contamination.
- Beta-lactamase testing and inducible resistance testing are not the same as routine susceptibility testing.
- CSF and other sterile-site specimens must be plated promptly; fastidious organisms such as Haemophilus and Neisseria need enriched media and added CO2.
- Routine stool culture has low yield in diarrhea; molecular panels and toxin testing are preferred for Clostridioides difficile.
Worked example
A blood culture bottle flags positive at 14 hours and the Gram stain shows gram-positive cocci in clusters. Subculture grows coagulase-positive staphylococci, and a cefoxitin disk shows resistance, confirming MRSA. Report MRSA explicitly and note that beta-lactam therapy is inappropriate.