Epidemiological validation of pulsed-field gel electrophoresis patterns for methicillin-resistant Staphylococcus aureus

yTo determine the stability of pulsed-field gel electrophoresis (PFGE) patt erns of methicillin-resistant Staphylococcus aureus in the nosocomial setti ng, we analyzed isolates from long-term carriers (>1 month) and from patien ts involved in well-defined nosocomial epidemics. The number of fragment di fferences between the first isolate and subsequent isolates in long-term ca rriers showed a bimodal distribution, with one group having 0 to 6 fragment differences and the other group having 14 to 24 fragment differences. The PFGE patterns of isolates involved in epidemics also presented a similar bi modal distribution of the number of fragment differences. Typing these isol ates with another molecular method (inter-IS256 PCR) showed that isolates o f the first group (i.e., with 1 to 6 fragment differences) were clonally re lated, whereas the second group (with 14 to 24 fragment differences) could be considered genetically different. Among long-term carriers with clonally related isolates, 74 of 84 (88%) of consecutive isolates showed indistingu ishable patterns, whereas 10 of 84 (12%) showed related patterns differing by one to six fragments. Moreover, the frequency of apparition of related p atterns is higher when the time between the first and the subsequent isolat e is longer. During seven nosocomial epidemics lasting from 1 to 15 months, only 2 of 120 isolates (1.7%) showed a pattern which was different, althou gh related, from the predominant one involved in each of these outbreaks.