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

• Published in: Applied microbiology and biotechnology Vol. 100; no. 13; pp. 5965 - 5976
• Main Authors: Hansen, Gunda, Johansen, Claus Lindvald, Marten, Gunvor, Wilmes, Jacqueline, Jespersen, Lene, Arneborg, Nils
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-07-2016; Springer; Springer Nature B.V
• Subjects: Acidification; Acids; Acids - metabolism; Analysis; Animals; Applied Microbial and Cell Physiology; Bacteria; Biomass; Biomedical and Life Sciences; Biotechnology; Cattle; Culture Media - chemistry; Culture Media - metabolism; Extracellular matrix; Fermentation; Flow Cytometry; Heterogeneity; Hydrogen-Ion Concentration; Lactic Acid - metabolism; Lactococcus; Lactococcus lactis; Lactococcus lactis - cytology; Lactococcus lactis - growth & development; Lactococcus lactis - metabolism; Lactose; Life Sciences; Microbial Genetics and Genomics; Microbial Viability; Microbiology; Milk - chemistry; Milk - metabolism; Observations; Physiological aspects; Physiology; Studies; Intracellular pH; Flow cytometry; Viability assessment; Acidification activity; Batch fermentation; Lactococcus lactis
• ISSN: 0175-7598; 1432-0614
• DOI: 10.1007/s00253-016-7454-3

In this study, we investigated the influence of three extracellular pH (pH
ex
) values (i.e., 5.5, 6.5, and 7.5) on the growth, viability, cell size, acidification activity in milk, and intracellular pH (pH
i
) of
Lactococcus lactis
subsp.
lactis
DGCC1212 during pH-controlled batch fermentations. A universal parameter (e.g., linked to pH
i
) for the description or prediction of viability, specific acidification activity, or growth behavior at a given pH
ex
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 pH
ex
, the acidification activity per cell decreased over time which seemed to be linked to cell shrinkage. Flow cytometric pH
i
determination demonstrated an increase of the averaged pH
i
level for higher pH set points, while the pH gradient (pH
i
–pH
ex
) and the extent of pH
i
heterogeneity decreased. Cells maintained positive pH gradients at a low pH
ex
of 5.5 and even during substrate limitation at the more widely used pH
ex
6.5. Moreover, the strain proved able to grow despite small negative or even absent pH gradients at a high pH
ex
of 7.5. The larger pH
i
heterogeneity at pH
ex
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 pH
i
heterogeneity at pH
ex
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.