Intracellular pH is a major factor in the induction of tolerance to acid and other stresses in Lactococcus lactis

Abstract

This study demonstrates that exposure of log-phase Lactococcus lactis subsp. cremoris 712 cells to mildly acid conditions induces resistance to normally lethal intensities of environmental stresses such as acid, heat, NaCl, H2O2, and ethanol. The intracellular pH (pH(i)) played a major role in the induction of this multistress resistance response. The pH(i) was dependent on the extracellular pH (pH(o)) and on the specific acid used to reduce the pH(o). When resuspended in fresh medium, cells were able to maintain a pH gradient even at pH(o) values that resulted in cell death. Induction of an acid tolerance response (ATR) coincided with an increase in the ability of cells to resist change to an unfavorable pH(i); nevertheless, a more favorable pH(i) was not the sole reason for the increased survival at acid PH(o). Cells with an induced ATR survived exposure to a lethal pH(o) much better than did uninduced cells with a pH(i) identical to that of the induced cells. Survival following lethal acid shock was dependent on the pH(i) during induction of the ATR, and the highest survival was observed following induction at a pH(i) of 5.9, which was the lowest pH(i) at which growth occurred. Increased acid tolerance and the ability to maintain a higher pH(i) during lethal acid stress were not acquired if protein synthesis was inhibited by chloramphenicol during adaptation.