Molecular epidemiology of transmissible gentamicin resistance among coagulase-negative staphylococci in a cardiac surgery unit

The prevalence of colonization of patients in a cardiac surgery unit with gentamicin-resistant, coagulase-negative staphylococci increased from 20% to 68% over a period of four years. Gentamicin resistance was found to be plasmid associated and transmissible from wild-type coagulase-negative staphylococcal donors to a Staphylococcus aureus recipient by filter mating (conjugative). These plasmids were present in isolates from 50 (74%) of 69 individuals examined. This figure included isolates from colonized patients, colonized staff, and patients with prosthetic valve endocarditis. A common restriction-endonuclease digestion pattern (pG02; 50 kilobases) was seen in only 19 (38%) of the 50 conjugative plasmids. However, filter hybridization, restriction-endonuclease mapping, and transposon insertional mutagenesis showed that representatives of the other 10 restriction-endonuclease digestion patterns were physically related to pG02 over greater than 80% of their genome, with differences largely due to deletions or insertions of DNA in three areas, and that their gentamicin-resistance genes were identical. Molecular analysis may be required to ascertain the physical similarity among phenotypically and epidemiologically related plasmids.