Molecular fingerprinting of mupirocin-resistant methicillin-resistant Staphylococcus aureus from a burn unit.

OBJECTIVES

To characterize mupirocin-resistant methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients in a burn unit by pulsed-field gel electrophoresis and plasmid contents.

METHODS

A total of 53 methicillin-resistant S. aureus, consisting of 48 mupirocin-resistant and 5 mupirocin-susceptible MRSA were compared by plasmid content and pulsed-field gel electrophoresis of Sma I digested genomic DNA.

RESULTS

Of the 48 mupirocin-resistant isolates, 39 expressed high-level, and 9 expressed low-level mupirocin resistance. Plasmids were detected in all of the 53 isolates; however, only the high-level mupirocin-resistant isolates contained a 38 kb-conjugative plasmid that encoded high-level mupirocin resistance. Pulsed-field gel electrophoresis divided the isolates into four patterns designated types I to IV. Forty-three isolates consisting of 34 high-level, 5 low-level mupirocin-resistant and 4 mupirocin-susceptible isolates defined the type-I pattern. Eight isolates, five high-level and three low-level mupirocin-resistant isolates had the type-II pulsed-field pattern. The type-III and type-IV pulsed-field patterns consisted of a single isolate each. The type-I and type-II pulsed-field patterns were related and only differed by four Sma I bands.

CONCLUSIONS

Results of typing the mupirocin-resistant MRSA from the burn unit with pulsed-field gel electrophoresis indicated that closely related MRSA clones previously circulating in the unit had acquired a high-level mupirocin-resistant plasmid, and spread aided by mupirocin use.