Methicillin-resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen causing severe morbidity and mortality in many hospitals in Europe, in the United States and worldwide. Once these organisms are introduced into an institution they are particularly difficult to eradicate. Colonization with MRSA is a major problem since 30%-60% of patients who become colonized eventually develop infection, and these patients serve as reservoir for further nosocomial spread. There are a number of difficulties associated with the isolation and identification of MRSA in clinical laboratories. As a consequence of the resistance of many of these organisms, a number of factors affect the expression of methicillin resistance, e.g. the expression of methicillin resistance is influenced both by environmental factors and cell genotype. Although heterogeneous expression of resistance is common, some MRSA strains have a much more homogeneous display of resistance and may even be more of a problem. Antibiotic resistance and phagotyping patterns vary between the strains. In addition to the laboratory problems, the hospital needs a system for the immediate recognition of new patients carrying new importation of MRSA. Moreover, the concept of borderline resistance (that is thought to be due to very high level β-lactamase production) is still poorly defined in many respects, with a number of biological, diagnostic and clinical questions still unanswered. There is some debate over whether to report such strains as susceptible or resistant. There is increasing evidence that the factors accounting for the reduced susceptibility of the borderline Staphylococcus aureus strains to penicillinase-resistant penicillins are probably more complex than originally believed.
Biomedical Reviews 1994; 3: 65-71.
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