Glycopeptides. The acquisition of glycopeptide resistance by enterococci has been an epidemiological and antimicrobial dilemma for the past 25 years. First described in 1988, glycopeptide-resistant enterococci (GRE) have since emerged as a major cause of nosocomial infections. The majority of GRE infections are attributed to E. faecium, although glycopeptide resistance occurs in E. faecalis and other enterococcus species as well. Currently in the United States, an estimated 30% of clinical enterococcus isolates are resistant to glycopeptides.56 Vancomycin acts by binding to the D-ala-D-ala terminus of the pentapeptide precursor, thereby inhibiting cell wall synthesis. Glycopeptide-resistant organisms modify these pentapeptide precursors, replacing the terminal D-ala with D-lac or D-ser. These modified cell wall precursors bind glycopeptides with 1,000-fold lower affinity than do normal precursors. To create the modified precursors at least seven enzymes are required. Using the VanA cassette as a model, these enzymes are VanA, H, X, Y, Z, R and S (Fig. 1). Initially, cellular pyruvate is converted to d-lactate by the VanH dehydrogenase. The VanA ligase then ligates D-ala to D-lac. Host enzymes ligate D-ala-D-lac to the tripeptide precursor, yielding the low affinity pentapeptide precursor. Full resistance to glycopeptides, however, requires not only construc- tion of the altered precursor, but also elimination of normal precursors.33,57,58 VanX hydrolyzes D-ala-D-ala to its constituent amino acids, which allows D-ala-D-lac to be the sole substrate for cell wall synthesis.59 VanY hydrolyzes the terminal D-ala from any normal pentapeptide precursor, rendering it useless for normal cell wall construction.60 The mechanism by which VanZ augments resistance is unknown, but when present it confers decreased susceptibility to teicoplanin.61 Additional open reading frames VanW and VanV have been described on the VanB operon; their functions also are not yet known.62 Expression of the genes for VanA, H, X, Y and Z are all regulated by VanR and VanS, a two-component sensor-transducer system that is part of the VanA operon within Tn1546. While the specific regulatory factors are not known, the presence of glycopeptides in the environment results in activation of VanS through autophosphorylation. Activated VanS then phosphorylates Figure 1. An illustration of the VanA resistance mechanism as it relates to normal cell wall synthesis. The top pathway denotes normal cell wall synthesis, and the mechanisms by which VanX and VanY disrupt this pathway. The shaded pathway denotes construction of a modified cell wall that is resistant to vancomycin. Adapted from Gold et al.58 www.landesbioscience.com Virulence 425