Methicillin-resistant Staphylococcus aureus.

On May 16, 1990, Division L (Nosocomial Infections) of the American Society for Microbiology (ASM) held a seminar on methicillin-resistant Staphylococcus aureus (MRSA) as part of the Society’s 90th annual meeting. The seminar was convened by Maury E. Mulligan, MD, and one of the editors (AIH). Subsequently, the speakers submitted manuscripts related to topics they presented. Because of the marked interest in and many controversies surrounding MRSA in hospital epidemiology circles, Infection Control and Hospital Epidemiology agreed to publish these papers. James H. Jorgensen, PhD, begins by describing the mechanism for methicillin resistance (a unique penicillin-binding protein, PBP 2a which has a very low affinity for all b-lactam antibiotics) that is identical in all strains of methicillin-resistant staphylococci. The differences between MRSA and other S aureus isolates less susceptible but not resistant to methicillin are reviewed. These other isolates have been described as borderline resistant (BORSA) and modified (MODSA) S aureus. BORSA and MODSA isolates do not contain PBP 2a. Furthermore, unlike MRSA, BORSA and MODSA are of as yet unknown epidemiologic significance, and clinically infected patients can and should be preferentially treated with B-lactam antibiotics. He concludes by emphasizing the need for clinical laboratories to use one of three standardized methods for the accurate detection of methicillin resistance in S aureus. A recently described DNA probe for the direct detection of PBP 2a may supplant these phenotypic tests of methicillin resistance in staphylococci when kit tests become commercially available.’ Maury E. Mulligan, MD, and Robert D. Arbeit, MD, review tests used for strain differentiation of MRSA isolates. Such tests may be of value to the epidemiologist interested in investigating the endemicity or epidemicity of specific MRSA strains. Alternatively, clinicians may use these tests to examine isolates from pretreatment and posttreatment cultures of patients. Identity of sequential isolates suggests failure or relapse, whereas differences between sequential isolates suggests a new infection or new colonization. The tests classically used for S aureus strain differentiation-antibiotic susceptibility patterns and bacteriophage typingare inadequate. MRSA strains can become resistant in vitro and in vivo to many antibiotics once exposed, and many MRSA isolates are nontypable when tested with the usually available phage sets. Among the newer methods, restriction endonuclease analysis of plasmid DNA (REAP DNA fingerprinting) has been most widely studied, is highly reproducible, has good discriminatory power, and is offered by some laboratories as a service test. Tests that may be even more effective in typing