Purpose: The purpose of this study is to determine the conditions inducing alkaline tolerance response in Lm1/2a and 4b strains and the stability of such stress-resistant phenotypes.
Methods: Lm cells were pre-exposed to pH 9.0 for 1 h at 37°C followed by post-exposure to lethal pH 11.5 at 4°C, 22°C and 37°C in TSBYE. The time taken to induce alkali-stress adaptation was demonstrated by pre-exposing cells at pH 9.0 for 5, 15, 30 and 60 min. The stability assay was carried out by transferring adapted cells to neutral broth for 5, 15, 30, 60 and 75 min prior to lethal alkali stress in pH 11.5 using 4M KOH or NaOH.
Results: Mild alkali stressed Lm cells (pH 9/1 h) exhibited greater than 3 log CFU/ml survival as compared to non-stressed cells at lethal alkali pH 11.5 for 8 h. Only 15 min of mild alkali-stress was sufficient to induce alkali-stress resistant phenotype in Lm Bug600 and ScottA. Alkaline-stress adaptation in Lm was reversible within 1.5 h when cells were transferred to neutral pH. Similar patterns of alkaline-stress adaptation of Lm was observed at 37°C (greater than ~ 4 logs CFU/ml survival) when cells were pre-exposed to mild KOH instead of NaOH.
Significance: The alkaline-stress resistant phenotypes of Lm were easily formed when cells were exposed to sublethal alkali-stress at different temperatures.