Nosocomial infections caused by staphylococci

Staphylococcus aureus and coagulase-negative species of staphylococci (CNS), particularly S epidermidis, are among the most frequently isolated bacteria from patients with nosocomial infection. Conventional methods used in the clinical microbiology laboratory for identification, susceptibility testing, and epidemiologic typing of staphylococci have several limitations Identification to species level is often limited to S. aureus, based on ability to produce coagulase, thermostable nuclease, and clumping factor. However, several newly-described species of staphylococci may express some of these characters, whereas atypical strains of S. aureus do not, leading to misidentification (1,2). Furthermore, among the 17 species of staphylococci currently recognized to be indigenous to humans, the potential of S. lugdunensis to cause serious infections is being increasingly recognized (1,2) Therefore, further speciation of CNS is clinically relevant. Distinction of true CNS infection from specimen contamination by skin flora can be addressed by demonstrating the repeated isolation of the same clone from multiple specimens, e g., blood cultures (1). Currently available, rapid biochemical test systems enable identification of staphylococci to species level with 60-90% accuracy (1). Antimicrobial susceptibility pattern, the only routinely available strain marker, is not reliable for clonal delmeation because of phenotypic variation among clonally derived isolates (1).