Plasmid-pattern analysis for the differentiation of infecting from noninfecting Staphylococcus epidermidis

Isolation of Staphylococcus epidermidis from cultures of blood was differentiated from culture contamination by the detection of identical isolates in two or more consecutive cultures from an infected patient. We used plasmid-pattern analysis as a tool for establishing the identity of individual isolates. In a control study of 15 patients with two or more cultures of blood contaminated with S. epidermidis, analysis revealed that none of the isolates had identical plasmid patterns. This reflected the variety of plasmid patterns among colonizing coagulase-negative staphylococci cultured from skin sites of uninfected patients. In contrast, plasmid-pattern identity was seen among sequential or paired S. epidermidis isolates from a given patient in 32 of 36 patients with documented S. epidermidis infection. The plasmid pattern of each set of isolates from patients was unique. Infections included prosthetic-valve endocarditis in 26 patients, cerebrospinal fluid-shunt or ventriculostomy infections in six patients, intravenous-catheter sepsis in two patients, urinary tract infection in one patient, and osteomyelitis in one patient. Plasmid-pattern analysis may therefore be useful in the diagnosis of S. epidermidis infections.