Alterations in peptidoglycan precursors and vancomycin susceptibility in Tn917 insertion mutants of Enterococcus faecalis 221

Abstract

Derivatives of the highly vancomycin-resistant Enterococcus faecalis strain 221 (MIC, 1,024 micrograms/ml) harboring Tn917 insertions in vanR, vanH, and vanA were compared with the parent strain and the susceptible plasmid-free strain JH2-2 (MIC, 2 micrograms/ml). Cytoplasmic pools of UDP-N-acetyl-muramyl-peptide precursors of strain 221 contained the depsipeptide-terminating precursor as well as elevated levels of both the tripeptide and tetrapeptide precursors. Insertional inactivation of vanR resulted in the loss of carboxypeptidase activity, full susceptibility to vancomycin, and precursor pools similar to those of JH2-2. For the vanA insertional mutant the MBC of vancomycin was fourfold higher than that for JH2-2, and the mutant had increased levels of tripeptide and tetrapeptide precursors compared with those for JH2-2. The vanH insertional mutant showed elevated levels of these precursors, as well as a small amount of depsipeptide, and both the MIC and the MBC of vancomycin were increased compared with those for JH2-2. These findings suggest that DD-carboxypeptidase activity, under the control of vanR, results in increased pools of both tripeptide and tetrapeptide precursors, which may contribute to survival in the presence of vancomycin.