Species-specific and ubiquitous DNA-based assays for rapid identification of Staphylococcus epidermidis

Staphylococcus epidermidis is an aerobic gram-positive coccus that is now recognized among the coagulase-negative staphylococci as an etiological agent with an important range of pathogenicity in humans. Several diagnostic kits based on biochemical or immunological reactions can efficiently identify Staphylococcus aureus. However, these tests are often unreliable for the identification of coagulase-negative staphylococcal species including S. epidermidis. Since DNA-based assays for the species-specific identification of S. epidermidis remain unavailable, we have developed such tests in order to improve the accuracy and the rapidity of tests for the diagnosis of S. epidermidis infections. On the basis of the results of hybridization assays with clones randomly selected from an S. epidermidis genomic library we identified a chromosomal DNA fragment which is specific and 100% ubiquitous for the identification of S. epidermidis. This 705-bp fragment was sequenced and used to design PCR amplification primers. PCR assays with the selected primers were also highly specific and ubiquitous for the identification from bacterial cultures of clinical isolates of S. epidermidis from a variety of anatomic sites. While three strains of S. capitis were misidentified as S. epidermidis with the API Staph-Ident system and 2.5% of the S. Epidermidis identifications were inconclusive with the MicroScan Autoscan-4 system, the PCR assay was highly specific and allowed for the correct identification of all 79 S. epidermidis strains tested. The PCR assays developed are simple and can be performed in about 1 h. These DNA-based tests provide novel diagnostic tools for improving the diagnosis of S. epidermidis infections.