Sensitivity of acid-adapted and acid-shocked Shigella flexneri to reduced pH achieved with acetic, lactic, and propionic acids

Survival and growth characteristics of unadapted, acid-adapted, and acid-shocked Shigella flexneri 2a cells in acidified (pH 3.5 to 5.5) tryptic soy broth with 0.25% glucose (TSB) and tryptic soy agar (TSA) were determined. S. flexneri was grown at 37°C for 18 h in tryptic soy broth without glucose (TSBNG) (unadapted) and TSBNG supplemented with 1% glucose (TSBG) (acid-adapted). Cells grown in TSBNG were acid shocked by adjusting 16-h cultures to pH 5.05 ± 0.05 with lactic acid. Cells were then inoculated into TSB acidified with acetic, lactic or propionic acids to pH 5.5, 4.5, or 3.5 and incubated at 37°C for 6 h. The order of lethality at a given pH was lactic acid < acetic acid < propionic acid. Significantly (P ≤ 0.05) higher numbers of acid-adapted cells, compared to acid-shocked and unadapted cells, were recovered from TSB acidified (pH 3.5) with lactic or acetic acids. None of the cells survived a 30-min exposure in TSB acidified with propionic acid to pH 3.5. When the three cell types were plated on TSA acidified with lactic, acetic, or propionic acids at pH ≤ 4.5, ≤5.5, and ≤5.5, respectively, visible colonies were not detected. Viable unadapted, acid-adapted, and acid-shocked cells were, however, recovered from TSA acidified with all three acids at pH ≥ 4.5. Acid-adapted and, to a lesser extent, acid-shocked cells survived at lower pH than did unadapted cells, indicating that prior exposure to mild acidic environment results in increased acid resistance. Survival of S. flexneri at a given pH was influenced by the type of acidulant used, a response characteristic exhibited by other gram-negative enteric pathogens.