A Staphylococcus aureus bacteraemia isolate shows a vancomycin E-test MIC of 2 mg/L yet the patient fails to defervesce on therapeutic dosing. Repeat testing by broth microdilution also reads 2 mg/L. Which confirmatory method best detects heteroresistant subpopulations in this setting?
- A Cefoxitin disk diffusion
- B Repeat E-test after 48 hours of incubation
- C Population analysis profile on brain-heart infusion agar containing graded vancomycin concentrations ✓
- D Latex agglutination for PBP2a
Correct answer: C. Population analysis profile on brain-heart infusion agar containing graded vancomycin concentrations
Explanation
Vancomycin heteroresistance (hVISA) means a small subpopulation grows at vancomycin concentrations above the MIC of the bulk population, so routine MIC methods read susceptible while treatment fails. The reference method is the population analysis profile: the isolate is plated on BHI agar with 0.5 to 4 mg/L vancomycin and the area under the recovery curve is compared with control strains Mu3 (hVISA) and Mu50 (VISA). Longer incubation or cefoxitin testing addresses methicillin resistance, not glycopeptide heteroresistance.
Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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