The D-alanine residues of Staphylococcus aureus teichoic acids alter the susceptibility to vancomycin and the activity of autolytic enzymes

Abstract

Recently, Staphylococcus aureus strains with intermediate resistance to vancomycin, the antibiotic of last resort, have been described. Multiple changes in peptidoglycan turnover and structure contribute to the resistance phenotype. Here, we describe that structural changes of teichoic acids in the cell envelope have a considerable influence on the susceptibility to vancomycin and other glycopeptides. S. aureus cells lacking D-alanine esters in teichoic acids exhibited an at least threefold-increased sensitivity to glycopeptide antibiotics. Furthermore, the autolytic activity of the D-alanine mutant was reduced compared to the wild-type, and the mutant was more susceptible to the staphylolytic enzyme lysostaphin. Vancomycin inhibited autolysis at very high concentrations but neither in the wild-type nor in the mutant was the autolytic activity influenced in the range of the MIC. Mutant cells had a considerably higher capacity to bind vancomycin.

FIG. 1

Zymographic analysis (A) and spontaneous autolysis with or without vancomycin (B) of S. aureus Sa113 wild type and dltA mutant. (A) Equal amounts of cell wall-associated proteins from S. aureus Sa113 wild type (lane 1) and from the isogenic dltA mutant (lane 2) were applied to an SDS-polyacrylamide gel containing heat-inactivated staphylococcal cells. Molecular mass standards are indicated in kilodaltons. (B) Equal amounts of cells were incubated without vancomycin (triangles), with 1 μg of vancomycin per ml (squares), or with 10 μg of vancomycin per ml (circles). The values are given as percentages of the initial A₆₀₀.

FIG. 2

Lysostaphin-mediated lysis of wild type (solid symbols) and dltA mutant (empty symbols). Equal amounts of cells were incubated without (squares) or with (circles) lysostaphin (1 μg/ml). The values are given as percentages of the initial A₆₀₀.