A COMPARISON OF METHODS TO DETERMINE WHETHER CLINICAL ISOLATES OF STAPHYLOCOCCUS-EPIDERMIDIS FROM THE SAME PATIENT ARE RELATED

Staphylococcus epidermidis is a major cause of hospital-acquired infec tions but also part of the normal skin flora. A common clinical questi on is whether repeated isolation of S. epidermidis from one patient re presents the same strain; because if different strains are isolated, t hey are often thought to be contaminants. In this study, different typ ing methods were compared to answer this question. Twenty isolates of S. epidermidis from five different patients were investigated. The iso lates from each patient had identical or very similar antibiograms, an d were recovered on different occasions. Typing was performed by antib iogram, biotype, slime production, plasmid profile, and pulsed-field g el electrophoresis (PFGE) banding pattern of SmaI digests of chromosom al DNA. In addition, the level of resistance to methicillin was determ ined by growth curves in broth containing methicillin for a series of different inocula for each isolate. The results showed that the isolat es from each patient belonged to the same clone, but examples of insta bilities in their antibiograms, plasmid profiles, as well as their PFG E banding patterns were seen. A change in the level of methicillin res istance was observed in one strain; otherwise this characteristic was found to be strain-specific and stable in vice. It was concluded that in combination with biotyping and antibiotic resistance testing the le vel of resistance to methicillin could be used as an aid to distinguis h between two or more clinical isolates of S. epidermidis from the sam e patient.