Genetics of glycopeptide resistance in enterococci

Glycopeptide resistance in enterococci is phenotypically and genotypically heterogeneous. The genes responsible for inducible resistance to high levels of vancomycin and teicoplanin (VanA phenotype) are carried by the 10,851-bp Tn1546 transposon. Transposition of Tn1546 into self-transferable plasmids and subsequent transfer by conjugation appears to be responsible for the dissemination of this type of resistance. Nine polypeptides are encoded by Tn1546 that belong to five functional groups: transposition functions (ORF1 and ORF2), regulation of resistance gene expression (VanR and VanS), synthesis of depsipeptide D-Ala-D-lactate (VanH and VanA), hydrolysis of D-Ala-D-Ala-containing peptidoglycan precursors (VanX and VanY), and low-level teicoplanin resistance (VanZ). VanB-type resistance (various levels of resistance to vancomycin and susceptibility to teicoplanin) is also due to production of D-Ala-D-Lac. The VanB ligase of VanB-type strains is structurally and functionally similar to VanA. The vanB gene was found on composite transposon Tn1547, which, in turn, was part of larger conjugative chromosomally located elements (90 to 250 kb). In contrast to acquired VanA- and VanB-type resistance, VanC-type resistance (low level of resistance to vancomycin and susceptibility to teicoplanin) is an intrinsic property of motile enterococci. Resistance in these species is due to synthesis of dipeptide D-Ala-D-Ser by VanC ligases leading to production of cell wall precursors with reduced vancomycin affinity.