MOLECULAR EPIDEMIOLOGY OF A MULTIPLE STRAIN OUTBREAK OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS AMONGST PATIENTS AND STAFF

Since methicillin-resistant Staphylococcus aureus (MRSA) isolates are not endemic in our hospital, which is a tertiary referral centre, the finding of 13 MRSA isolates from 12 patients associated with an acute vascular surgery ward between October 1993 and December 1993 prompted further epidemiological and laboratory investigations. Two strains wer e distinguished by antibiogram and phage-typing. One strain, resemblin g EMRSA-16, colonized six patients and was probably introduced from an other hospital in the Oxford Region. Five other patients were colonize d by a second strain, gentamicin-resistant and non-typable by phage-ty ping, probably introduced into the hospital 12 months previously by a patient from Nairobi, Kenya. A 12th patient was colonized by both stra ins simultaneously. Of 46 staff members screened three were colonized - one by an EMRSA-16 strain, a second by the gentamicin-resistant 'Nai robi'-strain a third member carried yet a further distinct MRSA strain . The healthcare worker colonized by the 'Nairobi'-strain had been car rying the isolate 12 months previously and was the likely source of th is strain. These isolates were also characterized by the repetitive ex tragenic palindromic-polymerase chain reaction (REP-PCR), a novel PCR- based methodology which has not been previously used in characterizing Staphylococcus aureus in an outbreak. This method corroborated the st rain classifications provided by the traditional methods, confirming t hat there had been spread of two strains simultaneously. Our study dem onstrates that multiple strains of MRSA may circulate amongst patients and staff during an outbreak, patients may be colonized by more than one strain simultaneously and long-term staff carriage (>12 months) ma y be an important source of colonization in patients. REF-PCR is a rap id and effective molecular typing method for MRSA.