Molecular characterization and LD(50) identify virulent isolates of Staphylococcus epidermidis from adult sepsis

Staphylococcus epidermidis plays an important role in infections of patients with implanted prosthetic devices. The exact clinical significance of recovered S. epidermidis from clinical specimens is difficult to assess, as they are inhabitants of the normal skin. In this study, 11 adults with clinical sepsis and blood cultures that grew only S. epidermidis were the host population. Bacterial virulence in vivo was determined by using the mouse LD(50) assay where the intravenous lethality was determined for each patient isolate. Bacterial dose (CFU x 10(9)) that produced lethality in 50% of the animals at 12 h was the value used for comparison. Restriction fragment length polymorphism (RFLP) analysis of chromosomal DNA by pulsed-field gel electrophoresis (PFGE) was used for identification of individual strains and their clonal organization. Confirmation of species assignment was done by RFLP analysis of 16S + 23S rRNA gene regions (ribotyping). Plasmid profile analysis was also conducted. Four of 11 blood isolates from adults with S. epidermidis sepsis had indistinguishable or closely related DNA patterns and were considered clone A. The same clone was previously seen to account for the majority of sepsis in a neonatal intensive care unit. There were significant differences in virulence characteristics of the S. epidermidis isolates. Clone A isolates produced lethality by LD(50) in mice at a dose averaging 2.35; clone B isolate at a dose of 2.54, and the remaining isolates, representing six distinct clones, were lethal to mice at significantly larger doses (3.51-5.17, average 4.16). These data suggest that individual clones of S. epidermidis isolated from septic adults have detectable differences in virulence as defined by an animal bioassay, and the more virulent clone is widespread.