Comparison of phage typing, plasmid mapping, and antibiotic resistance patterns as epidemiologic markers in a nosocomial outbreak of methicillin-resistant Staphylococcus aureus infections.

Isolates of methicillin-resistant Staphylococcus aureus obtained during a nosocomial outbreak were analyzed by phage typing, plasmid mapping, and antibiotic susceptibility patterns for epidemiologic markers. Antibiotic susceptibility patterns were of limited use epidemiologically because of multiple resistance and similarity of the strains. Phage typing demonstrated that the outbreak consisted of multiple introductions of MRSA organisms into the hospital from the community and from other hospitals, and of circulation of one predominant phage type, 6/47/54/81. Plasmid mapping further subdivided the organisms of this phage type into two different groups, one of which carried two particular plasmids. Organisms carrying these plasmids were significantly more resistant to methicillin and cephalosporins and were isolated from patients who had received prior antibiotic treatment.