Instability of antibiotic resistance in a strain of Staphylococcus epidermidis isolated from an outbreak of prosthetic valve endocarditis.

Plasmid profiles, phage typing, antibiograms, and biotyping were used to characterize Staphylococcus epidermidis isolated from multiple cultures of blood of four patients with prosthetic valve endocarditis. Epidemiological evidence implicated a common source for these infections. Of 20 clinically significant isolates, 14 exhibited variations from the prototype pattern of multiple resistance to five antibiotics. All isolates tested appeared to be the same strain by phage typing. Of 18 isolates available for plasmid analysis, 10 contained six plasmids of identical size, whereas eight differed from the prototype profile in the loss of one to three plasmids. Loss of resistance to gentamicin, chloramphenicol, erythromycin, and clindamycin but not to methicillin was associated with the loss of specific plasmids. Because antibiotic resistance in this strain of S. epidermidis was unstable, the use of antibiograms alone was not a reliable means of evaluating the relatedness of these multiple isolates.