Medical Microbiology
Microbiology questions test whether you can separate colonization from infection and select the test that answers the clinical question. Begin with specimen quality: an acceptable sputum Gram stain shows many neutrophils and few squamous epithelial cells, and a mixed-flora urine from a bag collection is uninterpretable. Know the diagnostic layers: direct Gram stain, culture with MALDI-TOF identification, and molecular methods (PCR, NAAT, whole-genome sequencing for outbreak typing). Remember the limitation that NAAT detects nucleic acid, not viability, so it generally cannot serve as a test of cure.
For susceptibility, interpret MICs against current breakpoints, recognize resistance mechanisms (mecA/PBP2a for MRSA, vanA/vanB for VRE, ESBL and carbapenemase producers, inducible AmpC), and understand that disk diffusion yields a categorical interpretation rather than a continuous value. Infection prevention concepts, including standard and transmission-based precautions, sterilization versus disinfection, and reportable diseases, appear regularly.
Traps
- Coagulase-negative staphylococci in one of four blood culture bottles usually reflect skin contamination.
- Clostridioides difficile NAAT positivity without diarrhea often represents colonization; test only unformed stool.
- A negative MRSA nasal swab has high negative predictive value for MRSA pneumonia but excludes nothing else.
- Serologic IgM alone is unreliable for acute diagnosis; use paired acute and convalescent sera.
Worked example. Draw two blood culture sets from separate sites, 20-30 mL per set, before antibiotics; this maximizes recovery. Growth of a gram-negative rod in under 24 hours indicates high-grade bacteremia and warrants urgent source evaluation.