A blood culture grows Enterococcus faecium with vancomycin MIC 128 microgram/mL and teicoplanin MIC 1 microgram/mL. PCR identifies the vanB gene cluster. Which statement best explains this phenotype?
- A vanB substitutes D-Ala-D-Ala termini and therefore spares teicoplanin
- B The teicoplanin result is falsely low because vanB strains always show heterogeneous teicoplanin resistance
- C Teicoplanin resistance requires vanC, which was not detected
- D vanB encodes D-Ala-D-Lac substitution but lacks functional expression of the vanHAX operon without induction, and its regulatory system does not respond to teicoplanin, leaving teicoplanin activity intact ✓
Correct answer: D. vanB encodes D-Ala-D-Lac substitution but lacks functional expression of the vanHAX operon without induction, and its regulatory system does not respond to teicoplanin, leaving teicoplanin activity intact
Explanation
Like vanA, vanB produces B-Ala-B-Lac terminating peptidoglycan precursors via VanH dehydrogenase and VanX B,B-dipeptidase. The difference lies in regulation: the vanB two-component system (VanRB-VanSB) is induced specifically by vancomycin-like glycopeptides and not by teicoplanin, so constitutive expression may be absent and teicoplanin retains activity. Option A is wrong on structure, and vanC confers intrinsic low-level resistance in E. gallinarum, not teicoplanin resistance.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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