A blood culture grows MRSA. The isolate's vancomycin MIC by E-test is 1.5 mg/L (susceptibility breakpoint: ≤2 mg/L). Clinical response is poor despite adequate vancomycin trough levels of 15-20 mg/L. Which concept best explains this treatment failure?
- A Heteroresistant VISA (hVISA) — a subpopulation of cells with higher vancomycin MIC (2-8 mg/L) not detected by standard MIC testing ✓
- B Inadequate duration; vancomycin requires 6 weeks minimum for bacteraemia
- C Vancomycin-resistant MRSA (VRSA) — high-level resistance transferred from VRE
- D Beta-lactamase-mediated inactivation of vancomycin
Explanation
Heteroresistant VISA (hVISA) is a well-recognised clinical phenomenon where the bulk population of S. aureus appears susceptible to vancomycin (MIC ≤2 mg/L) by standard tests, but a subpopulation (frequency 10⁻⁶ to 10⁻⁵) harbours cells with MIC 2-8 mg/L (VISA range). Under vancomycin pressure, these resistant subpopulations are selected, causing treatment failure. Detection requires population analysis profile/AUC (PAP-AUC) — the gold standard. VRSA (MIC ≥16 mg/L) carries vanA gene from VRE; vancomycin is not inactivated by enzymes. Vancomycin mechanism is not beta-lactam-related.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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