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

Abstract

To determine the stability of pulsed-field gel electrophoresis (PFGE) patterns of methicillin-resistant Staphylococcus aureus in the nosocomial setting, we analyzed isolates from long-term carriers (>1 month) and from patients involved in well-defined nosocomial epidemics. The number of fragment differences between the first isolate and subsequent isolates in long-term carriers 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 bimodal distribution of the number of fragment differences. Typing these isolates with another molecular method (inter-IS256 PCR) showed that isolates of the first group (i.e., with 1 to 6 fragment differences) were clonally related, 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 indistinguishable patterns, whereas 10 of 84 (12%) showed related patterns differing by one to six fragments. Moreover, the frequency of apparition of related patterns is higher when the time between the first and the subsequent isolate 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, although related, from the predominant one involved in each of these outbreaks.

FIG. 1

Examples of PFGE patterns observed in four epidemics. The asterisk denotes the most predominant patterns of the epidemic. The total number of fragments per pattern and the number of fragment differences compared to the epidemic pattern are indicated.

FIG. 2

Distribution of the number of fragment differences observed between PFGE patterns. A comparison of the first and subsequent MRSA isolates from long-term MRSA carriers (A) and a comparison between the epidemic pattern (most predominant) and other patterns recovered during epidemics (B) is shown.