Culture of tissue samples obtained peri-operatively is ‘the gold standard’ for determining the presence of infection in prosthetic revision surgery. The growth of identical bacterial strains in three or more specimens strongly indicates an infected prosthesis. With routine microbiological culture techniques, identification of different phenotypes of coagulase negative staphylococcus (CNS) will be interpreted as either contamination or a polybacterial infection. At our clinic, different phenotypes of CNS are cultured in approximately 20% of patients operated with a two-stage revision due to a chronic prosthetic infection.

We studied the genotype of different phenotypes of CNS cultured in specimens obtained from prosthetic joints.

We analysed 22 cases, where different phenotypes of CNS were cultured in tissue, and joint fluid specimens were collected peri-operatively. The pre-operative diagnosis was chronic prosthetic infection (n=16), aseptic revision (n=5) and primary prosthesis (n=1). Different phenotypes were assessed by colony morphology and/or antibiogram. Pulsed-field gel electrophoresis (PFGE) was employed to identify and compare the genotypes.

In 16 out of 22 cases (73 %), PFGE unveiled that phenotypically different strains of CNS belonged to the same genotype. Of these 16 cases 7 had different antibiograms. In the other group (6/22), phenotypically different strains of CNS did not belong to the same genotype. In the 16 cases with different phenotypes belonging to the same genotype, gentamicin bone cement had previously been used in 15 cases. In the other group (6/22), gentamicin bone cement had not been used previously in any case (p < 0.01, chi-square test).

Phenotypically different strains of CNS identified by routine microbiological techniques should not be classified readily as contamination or as a mixed bacterial infection in prosthetic surgery. A particular precaution should be taken in the case of patients who had previously been operated on with use of gentamicin-loaded bone cement.