Implications of acquired oxacillin resistance in the management and control of Staphylococcus aureus infections.

Refinements in testing for resistance to penicillinase-resistant penicillins (PRP) in Staphylococcus aureus have resulted in confusion in classifying isolates as PRP susceptible or resistant. Specifically, a group of organisms has been identified that produce large amounts of beta-lactamase and appear borderline resistant. These organisms have been called "occult resistant" or "acquired oxacillin-resistant" S. aureus (AORSA). A retrospective study was conducted to evaluate the implication of this in vitro phenomenon in managing patients with AORSA infections. Among 134 patients with S. aureus infections, 89 were infected with oxacillin-susceptible S. aureus (OSSA), 26 with AORSA, and 19 with oxacillin-resistant S. aureus (ORSA). There were no significant differences in outcomes when OSSA and AORSA infections were treated with PRP (chi 2MH = .990; P = .32). These results do not suppor the contention that AORSA infections should be managed differently from OSSA infections. Identifying AORSA may not be helpful in guiding antimicrobial therapy or predicting the outcome of infections with AORSA.