The adhesion of bacteria on a biomaterial surface is believed to be the first step in the development of biomaterial-related infection. The goal of this study was to investigate the mechanisms that permit adherent bacteria to persist on the surface of an implanted cardiovascular biomaterial. We hypothesized that circulating leukocytes are unable to adhere to the biomaterial surface under physiologic shear stress conditions, and this prevents them from interacting with adherent bacteria. To address this hypothesis, we investigated the adhesion profiles of Staphylococcus epidermidis and polymorphonuclear leukocytes (PMN), incubated under controlled shear stress conditions with the test biomaterial. We found that bacteria could adhere on the biomaterial surface, even when their concentration in the test medium was as low as 10(3) cfu/mL. At this concentration, the bacteria did not induce significant complement activation. PMN adhesion on the biomaterial surface was sensitive to shear stress and minimal at shear stress > 10 dynes/cm2. Low concentrations of bacteria could induce a significant increase in the expression of PMN adhesion molecules CD11b and CD11c. We conclude that the presence of bacteria induces PMN activation but does not increase PMN adhesion on biomaterial surfaces under physiologic shear stress conditions. This could be a major mechanism that protects adherent bacteria from PMN antibacterial activity.