Determination of the effect of supernatant fluids from oral streptococci on S. mutans CSP in agar well assays. Exogenous synthetic S. mutans CSP (final concentration, 2.5 μg/ml) was incubated with the supernatant fluids from oral streptococci at 37°C for 0.5 to 3 h. CSP thus treated was added to S. mutans GS5 comC mutant cells (107 CFU) in 1.0 ml of THBY to a final concentration of 0.25 μg/ml. After a 24-h incubation at 37°C, the supernatant fluids from the cultures were passed through 0.22-μm-pore-size filters, and the bacteriocin level produced by the S. mutans GS5 comC mutant was determined by the agar well assays. (A) Supernatant fluid from S. gordonii Challis. Part of the supernatant fluid was boiled for 10 min before the incubation with CSP. (B and C) PBS, a CSP positive control treated with PBS instead of supernatant fluid; zone 2, supernatant fluid from S. gordonii Challis mutant 41A9; zones 3 to 7, supernatant fluid from S. gordonii Challis, S. gordonii 10558, S. sanguis 10556, S. mitis 10712, and S. oralis KS32AR, respectively; and zone 8, supernatant fluid from S. mutans GS5. CSP was incubated with the supernatant fluid for 3 h in panels B and C.