Role of Penicillin-Binding Protein 5 in Expression of Ampicillin Resistance and Peptidoglycan Structure in Enterococcus faecium

Abstract

ABSTRACT The contribution of penicillin-binding protein 5 (PBP 5) to intrinsic and acquired β-lactam resistance was investigated by constructing isogenic strains of Enterococcus faecium producing different PBP 5. The pbp5 genes from three E. faecium clinical isolates (BM4107, D344, and H80721 ) were cloned into the shuttle vector pAT392 and introduced into E. faecium D344S, a spontaneous derivative of E. faecium D344 highly susceptible to ampicillin due to deletion of pbp5 (MIC, 0.03 μg/ml). Immunodetection of PBP5 indicated that cloning of the pbp5 genes into pAT392 resulted in moderate overproduction of PBP 5 in comparison to wild-type strains. This difference may be attributed to a difference in gene copy number. Expression of the pbp5 genes from BM4107 (MIC, 2 μg/ml), D344 (MIC, 24 μg/ml), and H80721 (MIC, 512 μg/ml) in D344S conferred relatively low levels of resistance to ampicillin (MICs, 6, 12, and 20 μg/ml, respectively). A methionine-to-alanine substitution was introduced at position 485 of the BM4107 PBP 5 by site-directed mutagenesis. In contrast to previous hypotheses based on comparison of nonisogenic strains, this substitution resulted in only a 2.5-fold increase in the ampicillin MIC. The reversed-phase high-performance liquid chromatography muropeptide profiles of D344 and D344S were similar, indicating that deletion of pbp5 was not associated with a detectable defect in cell wall synthesis. These results indicate that pbp5 is a nonessential gene responsible for intrinsic resistance to moderate levels of ampicillin and by itself cannot confer high-level resistance.