Evaluation of electrophoretic methods for typing met h i c i I I i n - resist a n t Staph yoco ccus aureus

Summary. Three electrophoretic methods of typing methicillin-resistant Staphylococcus aureus (MRSA) strains-plasmid profiles (PP), whole-cell protein profiles (WCPP) and immunoblotting profiles (1P)-were evaluated and compared with phage typing. The results obtained with isolates from 12 outbreaks were compared both within the outbreaks, to determine the consistency of results, and between outbreaks. There was generally good agreement between the typing methods but in only six outbreaks did all four methods indicate the same relationship between isolates. WCPP comprised more than 50 bands; when differences occurred, they were seen in only a few bands. In contrast, IP comprised only one or two major bands and the differences were much easier to interpret. The PPs of many of the isolates were similar; many isolates contained a plasmid of mol. wt (18-25) x lo6. In several outbreaks both WCPP and IP showed minor differences between isolates that were not apparent with phage typing. When comparisons were made between the 12 index strains and an isolate representing the London epidemic MRSA strain, phage typing and WCPP were the most discriminatory methods ; both gave nine distinct patterns, whereas there were eight IPS and only six PPs amongst the 13 strains. It was concluded that both WCPP and IP could provide valuable epidemiological data on MRSA and that IP was the easiest of the three methods to interpret.