Acid Tolerance Response of Listeria monocytogenes in Various External pHs with Different Concentrations of Lactic Acid

This study evaluated the acid tolerance response (ATR) of two strains of Listeria monocytogenes (serotype 1/2a and 4b) and one strain of Listeria innocua under different mildly acid conditions. Cells were incubated in combinations of three concentrations of lactic acid medium (3, 4.75, and 15 mM) and three external pH's (pH_ex 5.0, 6.0, and 6.5), plus, a HCl control, and a blank control (pH 7.4). Results showed that lactic acid induced lower log reduction of all three strains when challenged in severe acid conditions (pH 3.0) after being habituated at a pH_ex of 5.5 or 6.0 until the log phase, compared with a pH_ex of 6.5 or the two controls. This indicates that when the pH_ex was either 5.5 or 6.0 this induced a higher ATR of the strains, which may be caused by the ability of the strains to retain intracellular pH (pH_i) homeostasis with pH_i maintained in the range of 7.4-7.9. It was also found that a pH_ex of 5.5 resulted in the highest pH_i of the strains across all incubated conditions, which indicates that the pH_i may play an important role in the induction of ATR when Listeria cells are habituated in lactic acid, and if the higher pH_i can be maintained, the ATR would be stronger. The concentration of lactic acid per se has no significant effect on ATR, which it is proposed was due to the pH_i homeostasis maintained within the cells. However, the difference in ATR among three strains was also significant, which cannot be explained by the stable pH_i of all tested strains. Therefore, other underlying mechanisms to mediate ATR under different conditions need to be explored in further studies.