The clinical microbiology laboratory as an aid in infection control. The application of molecular techniques in epidemiologic studies of methicillin-resistant Staphylococcus aureus.

A microbiologic surveillance study was performed in order to estimate the point prevalence, source, and nosocomial acquisition of methicillin-resistant Staphylococcus aureus (MRSA) within the Iowa City Veterans Affairs Medical Center (IC VAMC). Immediately following the microbiologic surveillance study, a cluster of nosocomial MRSA infections was detected by routine infection control surveillance. An epidemiologic investigation was conducted and all isolates of MRSA detected during the microbiologic surveillance study and the subsequent cluster of nosocomial infections were characterized by restriction endonuclease analysis of plasmid DNA (REAP). REAP subtyping defined a total of ten distinct subtypes from 24 patients infected or colonized with MRSA. The documentation of a single subtype of MRSA (subtype A2) in nine patients from the surgical service, eight of which were hospitalized in the surgical intensive care unit, provided convincing evidence of a breakdown of infection control practices in that unit. REAP subtyping was a highly discriminating means of identifying different subtypes among the various isolates of MRSA and was useful in directing infection control efforts to specific problem areas within the hospital. Molecular typing methods, such as REAP, when used appropriately in conjunction with careful epidemiologic investigation provide an effective approach to the investigation and control of the spread of MRSA within the hospital.