Vancomycin Resistance in Enterococci Due to Synthesis of Precursors Terminating in d -Alanyl- d -Serine

Abstract

2Resistance to glycopeptides was not reported for approximately 30 years after the introduction of vancomycin into clinical practice, due partly to limited use of the antibiotic until the mid-1970s. The lengthy period was also due to the difficulties experienced by bacteria in developing mechanisms of resistance to an antibiotic which binds to an essential substrate in a biosynthetic pathway rather than to a protein or nucleic acid. Following publication in 1988 of reports of the first vancomycin-resistant enterococci (25, 39), it was predicted that resistance might arise by one of four possible routes, including inactivation of the antibiotic (not yet observed), sequestration of the antibiotic in the outer cell wall layers by specific and nonspecific binding (the probable mechanism by which lowlevel resistance is achieved in staphylococci), by an increased production of those intermediates to which vancomycin binds, or by a change in the target site (33). It was later recognized that the last mechanism would involve not only a new pathway to achieve a change of target but also elimination of at least part of the normal susceptible pathway (36).In spite of the complexity of such a mechanism, the enterococci have developed two similar strategies involving a change of target. High-level resistance in Enterococcus faecium and Enterococcus faecalis is characteristic of the VanA, VanB, and VanD types, in which the acyl-D-Ala-D-Ala terminus of peptidoglycan precursors to which glycopeptides bind has been replaced by acyl-D-Ala-D-lactate with the loss of a crucial hydrogen bond in the binding site. This results in a more-than-1,000-fold lowering of the affinity of vancomycin for its target. The substitution is achieved by the activity of two enzymes, a D-lactate dehydrogenase and a ligase with specificity directed towards synthesis of D-Ala-D-lactate(D-Lac) rather than D-Ala-D-Ala (16). An essential aspect of the resistance mechanism is the elimination of D-Ala-D-Ala by a DD-dipeptidase and/or of precursors containing this moiety by a DD-carboxypeptidase that removes the C-terminal D-Ala (8,36). This mechanism has been extensively reviewed (7,12,22,34,40,41).The second mechanism, which confers a low level of resistance to vancomycin only, involves replacement of D-Ala-D-Ala by a different dipeptide, D-Ala-D-Ser, rather than by the depsipeptide D-Ala-D-Lac (14,37). This mechanism is utilized by the intrinsically glycopeptide-resistant Enterococcus gallinarum, Enterococcus casseliflavus, and Enterococcus flavescens (VanC type) and is present in the VanE and VanG types in which E. faecalis has acquired the genes encoding vancomycin resistance. Some of the enzymes implicated in this second resistance mechanism are different from those involved in high-level resistance, indicating a second route by which resistance has evolved. The basis of resistance results from the sixfold-lower affinity of vancomycin for acyl-D-Ala-D-Ser than for acyl-D-Ala-D-Ala (13) because of the increased bulk of the hydroxymethyl group of serine re...

Antimicrobial Resistance and Multilocus Sequence Typing of Vancomycin-ResistantEnterococcus faeciumIsolated from the Chungcheong Area

“…VREs are classified into 6 categories depending on amino acid sequence of ligase. Among those, VanA and VanB have been frequently reported [15,16]. In general, VanA type E. faecium shows high resistance to vancomycin and teicoplanin.…”

Current Trends of Emergence and Spread of Vancomycin-Resistant Enterococci

“…All these resistance types require activity of a serine racemase converting L-Ser into D-Ser, the first one of these enzymes/genes was described in E. gallinarum (Arias et al, 1999). The vanC-1 gene cluster of E. gallinarum contains a ligase gene vanC-1, a combined D-Ala-D-Ala dipeptidase/carboxypeptidase vanXY C gene, a vanT racemase gene and two genes encoding a sensor kinase/response regular two-component regulatory system vanR C and vanS C Reynolds and Courvalin 2005). The vanC-2 cluster in E. casseliflavus showed a composition similar to the vanC-1 cluster in E. gallinarum (Dutta and Reynolds 2002).…”

Enterococcus gallinarum N04-0414 Harbors a VanD-Type Vancomycin Resistance Operon and Does Not Contain a d -Alanine: d -Alanine 2 ( ddl2 ) Gene

“…Low-level intrinsic resistance by VanC-type D-Ala:D-Ser ligases is found in Enterococcus gallinarum (VanC1), Enterococcus casseliflavus (VanC2), and Enterococcus flavescens (VanC3) (23). E. gallinarum has been previously associated with both VanA (13) and VanB (9).…”

“…The D-Ala:D-Ala ligase 2 gene (ddl2) was not present in the strain, though it is found downstream of the vanS gene from the vanC operon in E. gallinarum ATCC 49573 and 19 other E. gallinarum strains tested.Acquired glycopeptide resistance in enterococci can be mediated through three types of D-Ala:D-Lac ligases, VanA, VanB, and VanD, and two types of D-Ala:D-Ser ligases, VanE and VanG (3,8,23,24). Low-level intrinsic resistance by VanC-type D-Ala:D-Ser ligases is found in Enterococcus gallinarum (VanC1), Enterococcus casseliflavus (VanC2), and Enterococcus flavescens (VanC3) (23).…”