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Plasmids mediating high-level resistance to mupirocin (MIC > 1000 mg/ L) in staphylococci from various sources were studied by restriction endonuclease cleavage. Several patterns were obtained but six plasmids isolated from various Staphylococcus aureus and S. epidermidis strains were indistinguishable. The diversity and spread of these plasmids is illustrated.

Twenty-six nurses were repeatedly screened for carriage of epidemic methicillin-resistant Staphylococcus aureus (EMRSA) immediately before and after duty periods in which they solely attended six patients widely colonized with two EMRSA strains distinguishable by plasmid analysis. EMRSA carriage was detected in 13 nurses. Three EMRSA carriage patterns emerged: transient carriage in 12 nurses, when the EMRSA was isolated from noses or fingers of nurses after duty but was gone before their next day's duty; short-term nasal carriage, seen on occasion in 4 of these 12 nurses, when EMRSA carriage was detected on two consecutive screens; and persistent nasal carriage, seen in 1 nurse only, when the EMRSA was seen on more than two consecutive occasions. All but one of these incidents of carriage could be explained by close patient, rather than environmental, exposure and occurred despite an intensive control programme. Transient or short-term carriage in nurses probably resulted in transfer of the EMRSA between patients. Staff decontamination should be considered following a period of cohort nursing of EMRSA patients, especially if staff members are shortly to nurse unaffected patients. Our findings may explain some of the difficulties in controlling EMRSA.

SUMMARYThe spread of two strains of Staphylococcus aureus with high level resistance to mupirocin is described. The resistance proved to be easily transferred to other S. aureus strains by filter mating experiments and on the skin of mice. No plasmid band corresponding to the resistance could be demonstrated by agarose gel electrophoresis or by caesium chloride gradient centrifugation but cleavage of 'chromosomal' DNA from resistant recipients showed bright bands of DNA absent from sensitive controls.

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Summary. Probes constructed from a 4.05-kb EcoRI digest fragment of a mupirocin resistance plasmid and a 75 1-bp internal part of this fragment hybridised with DNA from all of 36 independent high-level mupirocin-resistant staphylococci tested from seven centres ; most were Staphylococcus aureus. In most instances the probes detected an EcoRI digest fragment of approximately 4 kb. Probes did not hybridise to DNA from low-level resistant strains, nor from strains sensitive to mupirocin.

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