FIG. 4.

Acid stress tolerance of S. mutans UA159 (•), ΔspxA (▪), ΔspxB (○), and ΔspxAB (□) strains. (A) Growth in BHI medium, pH 5.5, at 37°C determined using the Bioscreen C growth reader monitor. (B) Long-term survival assay. Aliquots of cultures were first plated on BHI agar when the stationary growth phase was reached (day 0). (C) Acid killing. Aliquots were plated on BHI agar upon suspension in 0.1 M glycine (pH 2.85) and each 30 min thereafter. The growth curve in panel A at pH 5.5 is a representative of at least five independent experiments, and the curves shown in panels B and C are the averages with standard deviations of the results from three independent experiments.