Mechanisms of glycopeptide resistance in enterococci.

Inducible resistance to high levels of glycopeptide antibiotics in clinical isolates of enterococci is mediated by Tn1546 or related transposons. Tn1546 encodes the VanH dehydrogenase which reduces pyruvate to D-lactate (D-Lac) and the VanA ligase which catalyses synthesis of the depsipeptide D-alanyl-D-lactate (D-Ala-D-Lac). The depsipeptide replaces the dipeptide D-Ala-D-Ala leading to production of peptidoglycan precursors which bind glycopeptides with reduced affinity. In addition, Tn1546 encodes the VanX dipeptidase and the VanY D,D-carboxypeptidase that hydrolyse the dipeptide D-Ala-D-Ala and the C-terminal D-Ala residue of the cytoplasmic precursor UDP-MurNAC-L-Ala-gamma-D- Glu-L-Lys-D-Ala-D-Ala, respectively. These two proteins act in series to eliminate D-Ala-D-Ala-containing precursors. VanX is required for resistance whereas VanY only slightly increases the level of resistance mediated by VanH, VanA and VanX.