Epidemiological typing of methicillin-resistant Staphylococcus aureus outbreak isolates by pulsed-field gel electrophoresis and antibiogram

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most commo n nosocomial pathogens. In April 1997, there were five MRSA-infected patien ts among 16 patients in the Neonatal Intensive Care Unit (NICU), Seoul Nati onal University Hospital, which is a tertiary-care hospital with 1,500 beds . The infections had spread from twin patients with MRSA who had transferre d from Hospital C. MRSA was isolated from the axilla of 15 (94%) of the 16 patients, including the two patients with obvious infections. Three (19%) o f 16 doctors and nine (30%) of 30 nurses had MRSA colonization of the anter ior nares. Six different PFGE patterns (A through F) were identified in the 53 isolates of MRSA tested. Twelve of 13 isolates from infected sites of f ive patients showed pattern F. Three MRSA strains obtained from hospital C showed closely or possibly related pattern F. MRSA of type F was isolated f rom three of 16 patients' axilla, and one of 3 doctors' and three of 30 nur ses' nasal swabs. The antibiogram code for 12 of 13 MRSA isolates from five infected patients Mns 66754. PFGE patterns of these isolates a ere either F, F1, F2 or Fa. Only one of three strains isolated from clinical specimens of patients in Hospital C showed the antibiogram code 66754, although they were all PFGE types F1 and Fa. In conclusion, the presumptive sources of t he outbreak of MRSA infection in NICU were the twin patients transferred fr om hospital C. Antibiogram correlated reasonably well to the PFGE type. An effective notification system is needed when a MRSA-infected patient is tra nsferred to another hospital to control the spread of the infection.