Mechanism and suppression of lysostaphin resistance in oxacillin-resistantStaphylococcus aureus

The potential for the development of resistance in oxacillin-resistant Stap hylococcus aureus (ORSA) to lysostaphin, a glycylglycine endopeptidase prod uced by Staphylococcus simulans biovar staphylolyticus, was examined in vit ro and in an in vivo model of infection. Following in vitro exposure of ORS A to subinhibitory concentrations of lysostaphin, lysostaphin-resistant mut ants were idenitifed among all isolates examined. Resistance to lysostaphin was associated with a loss of resistance to beta -lactams and a change in the muropeptide interpeptide cross bridge from pentaglycine to a single gly cine, Mutations in femA, the gene required for incorporation of the second and third glycines into the cross bridge, were found following PCR amplific ation and nucleotide sequence analysis. Complementation of lysostaphin-resi stant mutants with pBBB31, which encodes femA, restored the phenotype of ox acillin resistance and lysostaphin susceptibility. Addition of beta -lactam antibiotics to lysostaphin in vitro prevented the development of lysostaph in-resistant mutants. In the rabbit model of experimental endocarditis, adm inistration of a low dose of lysostaphin for 3 days led predictably to the appearance of lysostaphin-resistant ORSA mutants in vegetations. Coadminist ration of nafcillin with lysostaphin prevented the emergence of lysostaphin -resistant mutants and led to a mean reduction in aortic valve vegetation c ounts of 7.5 log(10) CFU/g compared to those for untreated controls and eli minated the isolation of lysostaphin-resistant mutants from aortic valve ve getations, Treatment with nafcillin and lysostaphin given alone led to mean reductions of 1.35 and 1.65 log(10) CFU/g respectively, In ORSA, resistanc e to lysostaphin was associated with mutations in femA, but resistance coul d be suppressed by the coadministration of beta -lactam antibiotics.