A spectrum of changes occurs in peptidoglycan composition of glycopeptide-intermediate clinical Staphylococcus aureus isolates

Abstract

The mechanism of glycopeptide resistance in Staphylococcus aureus is not known with certainty. Because the target of vancomycin is the D-Ala-D-Ala terminus of the stem peptide of the peptidoglycan precursor, by subjecting muropeptides to reversed-phase high-performance liquid chromatography, we investigated peptidoglycan obtained from glycopeptide-intermediate S. aureus (GISA) isolates for changes in composition and evaluated whether any peptidoglycan structural change was a consistent feature of clinical GISA isolates. GISA isolates Mu50 and Mu3 from Japan had the large glutamate-containing monomeric peak demonstrated previously, although strain H1, a vancomycin-susceptible MRSA isolate from Japan that was clonally related to Mu3 and Mu50, and a femC mutant that we studied, did also. For the U.S. GISA isolates, strain NJ had a large monomeric peak with a retention time identical to that described for the glutamate-containing monomer in strains H1, Mu3, and Mu50. However, a much smaller corresponding peak was seen in GISA MI, and this peak was absent from both GISA PC and a recent GISA isolate obtained from an adult patient in Illinois (strain IL). These data suggest that a uniform alteration in peptidoglycan composition cannot be discerned among the GISA isolates and indicate that a single genetic or biochemical change is unlikely to account for the glycopeptide resistance phenotype in the clinical GISA isolates observed to date. Furthermore, a large monomeric glutamate-containing peak is not sufficient to confer the resistance phenotype.

FIG. 1

Separation of muropeptides obtained by digestion with mutanolysin by reversed phase HPLC from vancomycin-susceptible MRSA and GISA isolates from Japan. (A) H1; (B) Mu50; (C) Mu50 (P15). M, monomeric; D, dimeric; T, trimeric.

FIG. 2

Separation of muropeptides obtained by digestion with mutanolysin by reversed phase HPLC from three GISA isolates obtained from patients in the United States. The GISA isolates are shown in the left hand panels, and the susceptible revertants of the clinical isolates are shown in the right hand panels. (A) NJ and NJ (P15); (B) MI and MI (P60); (C) PC and PC (P15).

FIG. 3

Separation of muropeptides obtained by digestion with mutanolysin by reversed-phase HPLC from two isolates obtained from the blood of a patient in Illinois. (A) MRSA, vancomycin-susceptible isolate; (B) genetically related GISA isolate.