FIG. 1.

Acid killing and hydrogen peroxide challenge assays. For acid tolerance, all S. mutans strains were grown in BHI until mid-exponential phase (OD₆₀₀ ≅ 0.3), harvested by centrifugation, washed once with 0.1 M glycine buffer, pH 7.0, and then subjected to acid killing at pH 2.8 for 45 min. For hydrogen peroxide challenge, the mid-exponential-phase cultures were incubated in buffer containing 0.2% (vol/vol) hydrogen peroxide for 2 h. The surviving cells were plated on BHI agar plates in triplicate, and results are expressed as survival rate over time. See text for more details. Data presented are means ± standard deviations (error bars) of three independent experiments. *, P < 0.01.