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 (pHex 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 pHex of 5.5 or 6.0 until the log phase, compared with a pHex of 6.5 or the two controls. This indicates that when the pHex 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 (pHi) homeostasis with pHi maintained in the range of 7.4-7.9. It was also found that a pHex of 5.5 resulted in the highest pHi of the strains across all incubated conditions, which indicates that the pHi may play an important role in the induction of ATR when Listeria cells are habituated in lactic acid, and if the higher pHi 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 pHi homeostasis maintained within the cells. However, the difference in ATR among three strains was also significant, which cannot be explained by the stable pHi of all tested strains. Therefore, other underlying mechanisms to mediate ATR under different conditions need to be explored in further studies.

中文翻译：

---

单核细胞增生李斯特氏菌在不同外部 pH 值和不同乳酸浓度下的耐酸反应。


本研究评估了两种单增李斯特菌菌株（血清型1/2a和4b）和一种无害李斯特菌菌株在不同弱酸条件下的耐酸反应（ATR）。将细胞在三种浓度的乳酸培养基（3、4.75 和 15 mM）和三种外部 pH（pHex 5.0、6.0 和 6.5）以及 HCl 对照和空白对照（pH 7.4）的组合中孵育。结果表明，与 pHex 6.5 或两个对照相比，在 pHex 5.5 或 6.0 习惯直至对数期后，在严酷的酸性条件（pH 3.0）下进行挑战时，乳酸诱导所有三种菌株的对数减少量较低。这表明当pHex为5.5或6.0时，这诱导了菌株较高的ATR，这可能是由于菌株保持细胞内pH（pHi）稳态（pHi维持在7.4-7.9范围内）的能力所致。还发现，在所有培养条件下，pHex 为 5.5 时菌株的 pHi 最高，这表明当李斯特菌细胞习惯于乳酸时，pHi 可能在 ATR 的诱导中发挥重要作用，并且如果 pHi 较高pHi能够维持，ATR就会更强。乳酸浓度本身对 ATR 没有显着影响，这被认为是由于细胞内维持 pHi 稳态所致。然而，三个菌株之间的 ATR 差异也很显着，这不能用所有测试菌株的稳定 pHi 来解释。因此，需要在进一步的研究中探索不同条件下介导 ATR 的其他潜在机制。