Staphylococcus epidennidis 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 vivo. 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.