In vitro assays of Staphylococcus epidermidis characteristics and outcome in an endocarditis model

Staphylococcus epidermidis adherence to indwelling polymers is important in prosthetic valve endocarditis. Earlier studies have related streptococcal endocarditis to isolates with high levels of cell-associated hexoses. The objective of the present study was to determine if a relationship exists between an S epidermidis isolate assay score and production/severity of experimental endocarditis.

Groups of patient S epidermidis isolates were screened for surface hexoses and an animal model of endocarditis with isolates testing highest and lowest on the screen was produced. Disease severity produced by 'high hexose' versus 'low hexose' organisms was evaluated. Endocarditis responding variables were bacterial vegetation weight and log(10) colony forming units (cfu) and in survival tests, comparative time to death with different isolates. Bacterial characteristics were not measured. Baseline data showed a vegetation weight difference so that with a β error of 0.20 and a two-tailed α error of 0.05, a significant difference would be noted using 30 animals. A total of 64 animals was used. POPULATION STUDDED: Bacterial isolates from two patient groups (n=42 and n=68) on which in vitro assays were run. An animal model of endocarditis (n=64) was used to evaluate four selected isolates for vegetation size, log(10) cfu/g, and survival time.

In a group of S epidermidis endocarditis animals evaluated for time of death, a significantly more rapid death time resulted in the group dosed with the high hexose-scoring organism (P<0.025). Vegetations and log(10) cfu produced by test high hexose isolates averaged larger but were not significantly different.

A significantly more rapid death rate occurs in untreated endocarditis using a high hexose isolate than with S epidermidis with low surface hexoses. Using bacterial vegetation and cfu as endpoints, however, experimental endocarditis using patient isolates of S epidermidis does not show the same strong correlation to bacterial surface hexoses as does streptococcal endocarditis.