Endocarditis due to methicillin-resistant Staphylococcus aureus in rabbits: expression of resistance to beta-lactam antibiotics

Whether a strain of methicillin-resistant Staphylococcus aureus susceptible to cephalo-thin by the disk-diffusion method was also susceptible to cephalothin in vivo was evalu-ated in the rabbit model of endocarditis. Rabbits with aortic-valve endocarditis due to methicillin-resistant S. aureus that were treated for four days with cephalothin had the same numbers of organisms in vegetations as did untreated rabbits. Treatment with cephalothin caused emergence of a highly resistant subpopulation in aortic-valve vegeta-tions. Organisms highly resistant to cephalothin were also highly resistant to nafcillin. Thus broth-dilution and disk-diffusion tests may not predict therapeutic failure for cephalothin against strains of methicillin-resistant S. aureus. Because of cross-resist-ances among f3-lactam drugs, these strains should be considered uniformly resistant to this general class of antimicrobial agents, regardless of results from these tests. Strains of methicillin-resistant Staphylococcus aureus (MRSA) have become established hospital flora and are important nosocomial pathogens [1, 2]. On the basis of results from susceptibility test