OBJECTIVE: To evaluate plasmid and chromosomal typing methods for differentiation of methicillin-resistant Staphylococcus aureus (MRSA). DESIGN: Comparison of relatedness of strains using epidemiologic features, phage typing, and antimicrobial susceptibility with blinded assessment by molecular typing methods. Molecular typing methods included AccI and ClaI restriction endonuclease fingerprinting of chromosomal DNA and plasmid typing. SETTING: Tertiary-care teaching hospital. METHODS: Convenience sample of 10 MRSA strains, including 4 outbreak-associated and 6 sporadic strains of diverse epidemiologic origins without evidence of nosocomial transmission. RESULTS: Only 2 strains were phage typeable. The epidemic strain was distinguishable by its susceptibility pattern. The other strains were not consistently separable by phenotyping or plasmid typing methods but were distinct by chromosome typing. CONCLUSIONS: These observations document the diversity of origins of MRSA strains introduced into an acute-care institution. Chromosomal typing may be the preferred method for the determination of clonal origin of MRSA.
OBJECTIVE: To evaluate plasmid and chromosomal typing methods for differentiation of methicillin-resistant Staphylococcus aureus (MRSA). DESIGN: Comparison of relatedness of strains using epidemiologic features, phage typing, and antimicrobial susceptibility with blinded assessment by molecular typing methods. Molecular typing methods included AccI and ClaI restriction endonuclease fingerprinting of chromosomal DNA and plasmid typing. SETTING: Tertiary-care teaching hospital. METHODS: Convenience sample of 10 MRSA strains, including 4 outbreak-associated and 6 sporadic strains of diverse epidemiologic origins without evidence of nosocomial transmission. RESULTS: Only 2 strains were phage typeable. The epidemic strain was distinguishable by its susceptibility pattern. The other strains were not consistently separable by phenotyping or plasmid typing methods but were distinct by chromosome typing. CONCLUSIONS: These observations document the diversity of origins of MRSA strains introduced into an acute-care institution. Chromosomal typing may be the preferred method for the determination of clonal origin of MRSA.