The bacterial stocks were stored at 80 C in tryptic soy broth (TSB; BectoneDickinson, Sparks, MD, USA) containing 20% (v⁄v) glycerol. Working cultures were prepared by propagating a stock culture through two consecutive 24 h growth cycles in TSB at 37 C. Cells were washed with sterile saline solution (0.85%, w⁄v), and the final pellets were resuspended in a volume corresponding to approximately 9 log CFU/ml.