Typing of consecutive methicillin-resistant Staphylococcus aureus isolates from intensive care unit patients and staff with pulsed-field gel electrophoresis.

Six consecutive methicillin-resistant Staphylococcus aureus (MRSA) isolates obtained in a 2-month period from tracheal aspirates of six intensive care unit (ICU) patients with nosocomial pneumonia and two MRSA isolates from nasal carriers among staff were typed to determine whether one or more strains were involved and whether nasal carriage was the source of the outbreak. Pulsed-field gel electrophoresis (PFGE) of chromosomal DNA was used to type MRSA isolates. The typing showed that the outbreak was caused by a single epidemic MRSA clone. The MRSA strain isolated from the staff was unrelated to the outbreak strain and was therefore not the source of the outbreak in this study. The source was apparently the index patient followed by transfer of MRSA to other patients on medical equipment or on the hands of staff who did not adhere strictly to infection control measures.