Influence of extracellular pH on growth, viability, cell size, acidification activity, and intracellular pH of Lactococcus lactis in batch fermentations

Abstract

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.

Influence of culture conditions on the technological properties ofCarnobacterium maltaromaticumCNCM I‐3298 starters

“…In their study, Hansen et al . () correlated low Lc. lactis acidification activities with decreasing cell sizes observed upon entry into stationary phase during fermentations carried out at pH 6·5 and 7·5, but not at pH 5·5.…”

“…lactis concentrates produced during batch fermentation (Hansen et al . ). Highest acidification activities were achieved at low pH (pH 5·5) and the negative effect of increasing harvest times during stationary phase was only seen at high pH (pH 6·5 and 7·5).…”

“…; Hansen et al . ). One of the main characteristics of Carnobacteria is their psychrotrophic nature and their ability to grow at high pH levels (Edima et al .…”

CFD predicted pH gradients in lactic acid bacteria cultivations

“…Langheinrich and Nienow (1999) reported pH gradients of 0.8 units due to alkali addition in an 8 m 3 reactor for mammalian cell cultures. The extracellular pH affects the intracellular pH of lactic acid bacteria (Cachon, Antérieux, & Diviès, 1998;Hansen et al, 2016) and by this the enzymatic activity. Aghababaie et al (2015) reported that the growth of S. thermophilus was reduced by 20% when cultivated 0.3 pH units away from the optimal pH conditions.…”

“…Aghababaie et al (2015) reported that the growth of S. thermophilus was reduced by 20% when cultivated 0.3 pH units away from the optimal pH conditions. In the immediate vicinity of the inlet for base addition, the cells are exposed to unfavorable pH values that might lead to viability loss (Hansen et al, 2016). Cortés et al (2016) and Amanullah et al (2001) showed in two-compartment scale-down studies of E. coli and B. subtilis, in which they mimicked oscillating pH conditions up to a delta pH of 0.9, that growth was not statistically significantly affected.…”