Equal volumes (10 ml ) of each culture were then combined, and cells were harvested by centrifugation at 4629×g (Eppendoef model 5810 R; Brinkmann Instruments Inc., Westbury, NY) for 15 min at 4 °C. The resulting cell pellet was washed twice with 60 ml of phosphate-buffered saline (PBS,pH 7.4;0.2 g/l KH2PO4,,1.5 g/l Na2HPO4•7H2O,8.0 g/l NaCl and 0.2 g/l KCl ), and after washing ,was rususpended in 60 ml PBS to provide a working culture with a concentration of approximately 9 log CFU/ml.