TYPING MULTIDRUG-RESISTANT STAPHYLOCOCCUS-AUREUS - CONFLICTING EPIDEMIOLOGIC DATA PRODUCED BY GENOTYPIC AND PHENOTYPIC METHODS CLARIFIED BYPHYLOGENETIC ANALYSIS

An outbreak of an unusual tetracycline-sensitive, rifampicin- and cipr ofloxacin-resistant, methicillin-resistant Staphylococcus aureus (MRSA ) strain at a large teaching hospital was investigated. Two typing met hods, phage typing and restriction fragment length polymorphism (RFLP) by pulsed-field gel electrophoresis (RFLP-PFGE), gave conflicting res ults which were clarified by phylogenetic analysis. Phage typing ident ified all the ''epidemic-associated'' strains as identical, while RFLP -PFGE further divided these strains into four pulsotypes. Phylogenetic analysis showed these four pulsotypes were related genetically and al so recognized a second strain of MRSA causing a continuing cross-infec tion problem. Variation in the RFLP-PFGE pattern was shown to occur fo llowing lysogenization of phage-sensitive MRSA. These results indicate that in analyzing outbreaks caused by subgroups of clonal organisms l ike MRSA, it is necessary to use at least two typing methods and that conflicts between these could be resolved by phylogenetic analysis.