The culture was centrifuged at 6,000 × g (Allegra 25R, Beckman
Coulter, Fullerton, CA) for 5 min and the supernatant was
decanted. The pellet was resuspended in 0.1 M phosphate
buffer saline solution (PBS) and the bacterial concentration was
adjusted to an optical density of 0.3 at 600 nm (≈1 × 108 CFU/ml)
spectrophometrically (Spectronic 20; Bausch and Lomb, Rochester,
NY) and diluted to a final concentration of ≈1 × 106 CFU/ml
in PBS.