title: 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.
authors.affiliation: Danisco Deutschland GmbH, Busch-Johannsen-Straße 1, 25899, Niebüll, Germany.
authors.forename: Gunda
authors.initials: G
authors.lastname: Hansen
authors.affiliation: DuPont Nutrition Biosciences ApS, Edwin Rahrs Vej 38, 8220, Brabrand, Denmark.
authors.forename: Claus Lindvald
authors.initials: CL
authors.lastname: Johansen
authors.affiliation: Danisco Deutschland GmbH, Busch-Johannsen-Straße 1, 25899, Niebüll, Germany.
authors.forename: Gunvor
authors.initials: G
authors.lastname: Marten
authors.affiliation: Danisco Deutschland GmbH, Busch-Johannsen-Straße 1, 25899, Niebüll, Germany.
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Wilmes
authors.affiliation: Department of Food Science, Food Microbiology, University of Copenhagen, Rolighedsvej 26, 1958, Frederiksberg C, Denmark.
authors.forename: Lene
authors.initials: L
authors.lastname: Jespersen
authors.affiliation: Department of Food Science, Food Microbiology, University of Copenhagen, Rolighedsvej 26, 1958, Frederiksberg C, Denmark. na@food.ku.dk.
authors.forename: Nils
authors.initials: N
authors.lastname: Arneborg
mesh_terms: D000143:Acids / Q000378:metabolism; D000818:Animals; D002417:Cattle; D003470:Culture Media / Q000737:chemistry / Q000378:metabolism; D005285:Fermentation; D005434:Flow Cytometry; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000378:metabolism; D013294:Lactococcus lactis / Q000166:cytology / Q000254:growth & development* / Q000378:metabolism*; D050296:Microbial Viability; D008892:Milk / Q000737:chemistry / Q000378:metabolism
chemical_list: D000143:Acids; D003470:Culture Media; D019344:Lactic Acid
keywords: Acidification activity; Batch fermentation; Flow cytometry; Intracellular pH; Lactococcus lactis; Viability assessment