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

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...

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Bibliographic Details
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
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Summary:	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.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	0175-7598 1432-0614
DOI:	10.1007/s00253-016-7454-3