Then 150 mL of the supernatant(5900 g, 20 min) from the 20 h LAB liquid culture was added into the Oxford cup and the plates were incubated at 37 C for 36 h. MRS medium, whose pH was adjusted to 3.2 with hydrochloric acid, was used as the negative control, and gentamicin (320 mg/ml; Sangon, Shanghai, China) was used as the positive control in each plate.