Danisco Deutschland GmbH, Busch-Johannsen-Straße 1, 25899, Niebüll, Germany.
DuPont Nutrition Biosciences ApS, Edwin Rahrs Vej 38, 8220, Brabrand, Denmark.
Appl Microbiol Biotechnol. 2016 Jul;100(13):5965-76. doi: 10.1007/s00253-016-7454-3. Epub 2016 Mar 29.
In this study, we investigated the influence of three extracellular pH (pHex) values (i.e., 5.5, 6.5, and 7.5) on the growth, viability, cell size, acidification activity in milk, and intracellular pH (pHi) of Lactococcus lactis subsp. lactis DGCC1212 during pH-controlled batch fermentations. A universal parameter (e.g., linked to pHi) for the description or prediction of viability, specific acidification activity, or growth behavior at a given pHex was not identified. We found viability as determined by flow cytometry to remain high during all growth phases and irrespectively of the pH set point. Furthermore, regardless of the pHex, the acidification activity per cell decreased over time which seemed to be linked to cell shrinkage. Flow cytometric pHi determination demonstrated an increase of the averaged pHi level for higher pH set points, while the pH gradient (pHi-pHex) and the extent of pHi heterogeneity decreased. Cells maintained positive pH gradients at a low pHex of 5.5 and even during substrate limitation at the more widely used pHex 6.5. Moreover, the strain proved able to grow despite small negative or even absent pH gradients at a high pHex of 7.5. The larger pHi heterogeneity at pHex 5.5 and 6.5 was associated with more stressful conditions resulting, e.g., from higher concentrations of non-dissociated lactic acid, while the low pHi heterogeneity at pHex 7.5 most probably corresponded to lower concentrations of non-dissociated lactic acid which facilitated the cells to reach the highest maximum active cell counts of the three pH set points.
在这项研究中，我们研究了三种细胞外 pH 值（即 5.5、6.5 和 7.5）对 Lactococcus lactis subsp. lactis DGCC1212 在 pH 控制分批发酵过程中的生长、活力、细胞大小、乳酸化活性和细胞内 pH（pHi）的影响。没有发现用于描述或预测给定 pHex 下活力、特定酸化活性或生长行为的通用参数（例如与 pHi 相关的参数）。我们发现，通过流式细胞术确定的活力在所有生长阶段都保持较高水平，且与设定的 pH 值无关。此外，无论 pHex 如何，每个细胞的酸化活性随时间的推移而降低，这似乎与细胞收缩有关。流式细胞术 pHi 测定表明，随着设定 pH 值的升高，平均 pHi 水平增加，而 pH 梯度（pHi-pHex）和 pHi 异质性程度降低。在低 pHex（5.5）下，细胞维持低 pHex 时能保持正的 pH 梯度，甚至在更广泛使用的 pHex 6.5 下底物限制时也能保持正的 pH 梯度。此外，尽管在高 pHex 7.5 下存在小的负 pH 梯度或甚至不存在 pH 梯度，该菌株仍能生长。在 pHex 5.5 和 6.5 时 pHi 异质性较大与更具压力的条件有关，例如更高浓度的未解离乳酸，而在 pHex 7.5 时 pHi 异质性较低，可能与较低浓度的未解离乳酸有关，这使得细胞能够达到三个 pH 设定点中最高的最大活性细胞计数。
