The potential for the development of resistance in
oxacillin-resistant Staphylococcus aureus (ORSA) to
lysostaphin, a
glycylglycine endopeptidase produced by Staphylococcus simulans biovar staphylolyticus, was examined in vitro and in an in vivo model of
infection. Following in vitro exposure of ORSA to subinhibitory concentrations of
lysostaphin,
lysostaphin-resistant mutants 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
glycine. Mutations in femA, the gene required for incorporation of the second and third glycines into the cross bridge, were found following PCR amplification and nucleotide sequence analysis. Complementation of
lysostaphin-resistant mutants with pBBB31, which encodes femA, restored the phenotype of
oxacillin resistance and
lysostaphin susceptibility. Addition of
beta-lactam antibiotics to
lysostaphin in vitro prevented the development of
lysostaphin-resistant mutants. In the rabbit model of experimental
endocarditis, administration of a low dose of
lysostaphin for 3 days led predictably to the appearance of
lysostaphin-resistant ORSA mutants in vegetations. Coadministration of
nafcillin with
lysostaphin prevented the emergence of
lysostaphin-resistant mutants and led to a mean reduction in aortic valve vegetation counts of 7.5 log(10) CFU/g compared to those for untreated controls and eliminated the isolation of
lysostaphin-resistant mutants from aortic valve vegetations. Treatment with
nafcillin and
lysostaphin given alone led to mean reductions of 1.35 and 1.65 log(10) CFU/g respectively. In ORSA, resistance to
lysostaphin was associated with mutations in femA, but resistance could be suppressed by the coadministration of
beta-lactam antibiotics.