Role of monocytes and bacteria in Staphylococcus epidermidis endocarditis

The endocardial vegetation which is formed in the course of bacterial endocarditis (BE) contains tissue factor (TF)-dependent procoagulant activity. Earlier studies showed that monocytes are the main source of TF in the vegetations. The TF activity (TFA) of vegetations isolated from Streptococcus sanguis-infected rabbits depended on the numbers of bacteria as well as monocytes in the vegetation. In this study, we investigated whether for Staphylococcus epidermidis, a frequent pathogen in BE, an effect similar to that found for S. sanguis could be shown. In vitro, S. epidermidis was found to stimulate TFA of fibrin adherent monocytes significantly. This stimulation was maximal at a bacterium-to-monocyte ratio of 7. In vivo, TFA was found to be significantly higher in S. epidermidis-infected than in sterile catheter-induced vegetations. Reduction of vegetational bacterial numbers by teicoplanin treatment lead to a small but significant decrease of TFA. Reduction of monocyte numbers by etoposide did not affect vegetational TFA. Comparison of data for S. epidermidis and S. sanguis revealed that at equivalent bacterial numbers, vegetational TFAs were approximately the same for both microorganisms. Combining the results of the present study with those of a previous study using S. sanguis, we conclude that the main factor determining monocyte-dependent vegetational TFA is the number of vegetation-associated bacteria. The lower TFA found for S. epidermidis-infected than for S. sanguis-infected vegetations can be explained by the significantly lower bacterial numbers in the infected vegetations and consequently a lower stimulation of vegetation-associated monocytes.