FIG. 4.
Acid tolerance assay. (A) S. gordonii carrying ParcA-cat and its derivatives were grown in BHI medium adjusted to pH 7.0 to an OD600 of 0.3, washed with 0.1 M glycine buffer (pH 7.0), and subjected to acid killing by incubating the cells in 0.1 M glycine buffer (pH 2.8). (B) The wild type and mutants of S. gordonii were grown in BHI medium adjusted to pH 7.0 to an OD600 of 0.2, and then the cells were harvested and resuspended in fresh BHI medium adjusted to pH 5.0. After two additional hours of incubation, cells with an OD600 of 0.3 were prepared for acid killing as described above. In all cases, the survival rate was determined by plating cells in triplicate on BHI agar plates. The results are expressed as the percent survival rate versus the time at pH 2.8. The data presented are representative of at least nine individual replicates for each strain.