FIG. 2.

(A) Flow cytometric analysis of S. macedonicus acid tolerance at mid-log phase. Nonadapted (a) or acid-adapted (b to f) cells, after exposure to pH 3.5 for 15 min, were labeled with cFDA and PI and analyzed by flow cytometry. Prior to acid challenge, bacteria were adapted either by autoacidification (b), transient exposure to pH 6.0 and 5.5 (c and d), or acid habituation at pH 6.0 (e). Acid habituation was also reversed after regrowth of habituated cells in MRS-MOPS (f). As additional controls, unstressed (g) and heat-killed (h) mid-log-phase cells were analyzed in parallel. (B) Percentages (means ± standard deviations) of fluorescent subpopulations of the samples presented in panel A. (C) Percentages (means plus standard deviations) of fluorescent subpopulations of samples challenged at pH 3.5 for 30 min in the same order as that presented in panel A.