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 infections but also part of the normal skin flora. A common clinical question is whether repeated isolation of S. epidermidis from one patient represents the same strain; because if different strains are isolated, they are often thought to be contaminants. In this study, different typing methods were compared to answer this question. Twenty isolates of S. epidermidis from five different patients were investigated. The isolates from each patient had identical or very similar antibiograms, and were recovered on different occasions. Typing was performed by antibiogram, biotype, slime production, plasmid profile, and pulsed-field gel electrophoresis (PFGE) banding pattern of SmaI digests of chromosomal DNA. In addition, the level of resistance to methicillin was determined by growth curves in broth containing methicillin for a series of different inocula for each isolate. The results showed that the isolates from each patient belonged to the same clone, but examples of instabilities in their antibiograms, plasmid profiles, as well as their PFGE banding patterns were seen. A change in the level of methicillin resistance 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 level of resistance to methicillin could be used as an aid to distinguish between two or more clinical isolates of S. epidermidis from the same patient.