Inducible and constitutive expression of vanC-1-encoded resistance to vancomycin in Enterococcus gallinarum

Clinical isolates Enterococcus gallinarum AIB39 and E. gallinarum GS1 were studied to establish whether the expression of vanC-1-mediated resistance may be inducible or constitutive. By growth curve analysis, strain AIB39 exhibited the same lag period (i.e., 1 to 1.5 h) whether it was subcultured to unsupplemented brain heart infusion broth or broth containing 6 micrograms of vancomycin per ml, a growth pattern typical of constitutively expressed resistance. Use of high-performance liquid chromatography (HPLC) to separate peptidoglycan precursor extracts substantiated this finding because the serine-terminating pentapeptide precursor UDP-MurNAc-L-Ala-D-Glu-L-Lys-D-Ala-D-Ser was produced in the presence and absence of vancomycin, whereas no UDP-MurNAc-L-Ala-D-Glu-L-Lys-D-Ala-D-Ala was detected. In contrast, results with strain GS1 were consistent with inducible expression. GS1 demonstrated a lag time that was 3 to 4 h longer when it was subcultured to vancomycin-containing broth than when it was subcultured in unsupplemented broth. HPLC analysis showed that in the absence of vancomycin only UDP-MurNAc-L-Ala-D-Glu-L-Lys-D-Ala-D-Ala was detected, but in the presence of drug only UDP-MurNAc-L-Ala-D-Glu-L-Lys-D-Ala-D-Ala was found. Inducible expression of vanC-1-mediated resistance in E. gallinarum is consistent with recent findings suggesting the presence of at least two ligases in this species. Although vanC-1 may be intrinsic to E. gallinarum, our findings raise doubt regarding the natural mechanism of this gene's expression.