Simplified method for the isolation, identification, and characterization of Staphylococcus epidermidis in epidemiologic studies

A simplified method for the isolation, identification, and characterization of Staphylococcus epidermidis from humans is described. Swabs of the nose and skin are cultured on mannitol salt agar. Isolated colonies not producing acid from mannitol (presumptive coagulase-negative staphylococci or micrococci) are then inoculated onto purple agar containing erythromycin and glycerol. All colonies growing on this medium are then replicated onto media that tests for the production of phosphatase, the production of acid from trehalose, and susceptibility to four antibiotics. All S. epidermidis sensu stricto are confirmed by the API Staph-Ident system. As a result, Staphylococcus aureus and all other coagulase-negative staphylococci are effectively identified and eliminated from further study and only strains of S. epidermidis are left for further characterization. Of the 252 isolates from 48 cultures of the nares and the fingers, 112 (44%) were eliminated during different stages of this isolation and identification procedure. The antibiotic susceptibility data further distinguished those isolates in the predominant API biochemical profile number. This scheme has applications in the early stages of either ecologic or epidemiologic studies of this important nosocomial pathogen.