Background: Methicillin-resistant Staphylococcus aureus (MRSA) continues to be a major cause of nosocomial infection and a molecular typing is necessary for proper epidemiologic investigations of sources and modes of spread in an outbreak. An nosocomial outbreak of MRSA in a neonatal intensive care unit at Ewha Womans University Mokdong Hospital was suspected. To investigate the clonality of isolates and control the spread of nosocomial outbreak, we performed plasmid restriction analysis of MRSA isolates from patients and medical staffs.

Methods: We studied 7 MRSA strains (umbilicus 4, blood 1, urine 1 and pus 1) from patients in a neonatal intensive care unit and the MRSA strains from nares and hands surveillance cultures of 26 medical staffs (4 medical doctors and 22 nurses). All MRSA strains were tested for antimicrobial susceptibility and plasmid analysis after EcoRI restriction. We analyzed the plasmid patterns of MRSA isolated from patients and compared with those from medical staffs.

Results: Ten MRSA strains (from 7 nares and 3 hands) were isolated from surveillance cultures of 26 medical staffs. Seven out of 10 MRSA strains from medical staffs revealed identical pattern of antibiogram which was the same pattern in all 7 MRSA strains from seven patients. Plasmid restriction patterns were classified 6 groups from A to F showing 2-10 bands. Six out of 7 MRSA strains from the patients showed group A(A1 5, A3 1) and 5 out of 10 MRSA strains from the medical staffs showed group A(A1 1, A2 1, A3 2, A4 1) and remainders showed different plasmid restriction analysis patterns.

Conclusions: These results suggest that plasmid restriction analysis is a rapid, inexpensive, and good discriminating molecular typing of MRSA outbreak and is useful for the epidemiologic investigation of MRSA outbreaks in the clinical laboratory. (Korean J Clin Microbiol 1999;2:125-130)