Inactivated pbp4 in highly glycopeptide-resistant laboratory mutants of Staphylococcus aureus

Both vancomycin- and teicoplanin-resistant laboratory mutants of Staphylococcus aureus produce peptidoglycans of altered composition in which the proportion of highly cross-linked muropeptide species is drastically reduced with a parallel increase in the representation of muropeptide monomers and dimers (Sieradzki, K., and Tomasz, A. (1997) J. Bacteriol. 179, 2557-2566; and Sieradzki, K,, and Tomasz, A. (1998) Microb, Drug Resist. 4, 159-168). We now report that the distorted peptidoglycan composition is related to defects in penicillin-binding protein 4 (PBP4); no PBP4 was detectable by the fluorographic assay in membrane preparations from the mutants, and comparison of the sequence of pbp4 amplified from the mutants indicated disruption of the gene by two types of abnormalities, a 17-amino acid long duplication starting at position 305 of the pbp4 gene was detected in the vancomycin-resistant mutant, and a stop codon was found to be introduced into the pbp4 KTG motif at position 261 in the mutant selected for teicoplanin resistance. Additional common patterns of disturbances in the peptidoglycan metabolism of the mutants are indicated by the increased sensitivity of mutant cell walls to the M1 muramidase and decreased sensitivity to lysostaphin, which is a reversal of the susceptibility pattern of the parental cell walls. Furthermore, the results of high performance liquid chromatography analysis of lysostaphin digests of peptidoglycan suggest an increase in the average chain length of the glycan strands in the peptidoglycan of the glycopeptide-resistant mutants. The increased molar proportion of muropeptide monomers in the cell wall of the glycopeptide-resistant mutants should provide binding sites for the "capture" of vancomycin and teicoplanin molecules, which may be part of the mechanism of glycopeptide resistance in S. aureus.