Staphylococcus epidermidis is a commensal of human skin flora and a frequent causative micro-organism in prosthetic joint infections (PJIs). To date, no single marker has been identified to distinguish infecting strains from commensal S. epidermidis populations.

To find possible genetic markers to distinguish between the two populations.

Fifty S. epidermidis strains from patients with PJIs were analysed, 50 from the skin of healthy individuals (commensal strains) and 17 from the surgical field of patients undergoing primary arthroplasty. In these three groups the antimicrobial susceptibility profile, sequence type, biofilm formation, and virulence factors were studied. Strains from the surgical field have not been compared previously with strains from the other two groups.

S. epidermidis strains from PJI patients were significantly more antibiotic resistant than commensal strains and surgical field strains. A wide variety of sequence types was found in commensal and surgical field strains. The predominant sequence type was ST2 and it was only present in PJI strains (44%). Differences in biofilm production did not differ between populations. Virulence genes sdrF and bhp, the complete ica operon, and the insertion sequence IS256 were significantly predominant in PJI strains. By contrast, embp and hld genes and the arginine catabolic mobile element (ACME) were more prevalent in commensal strains. Surgical field strains could be a valid control group to discriminate between infecting and commensal strains.

A combination of characteristic features can differentiate between infecting and commensal S. epidermidis strains in PJI, whereas a single marker cannot.