Aline (PBS), and diluted (final, concentration 5 107 CFU/ml) in PBS. Next, 200- l bacterial suspensions were mixed with 200 l of plasma or buffer, followed by incubation for 3 h in human plasma. After three further washing steps with PBS, the pellet was suspended in Tris-NaCl buffer (19.2 mM/1.062 M; pH 7.5) containing 20 g/ml of the chromogenic substrate S-2251 (H-D-Val-Leu-Lys-pNA?HCl; Sigma), f