Abstract
During storage and ripening of fermented foods, Lactococcus cremoris is predominantly in a non-growing state. L. cremoris can become stationary due to starvation or acidification, and its metabolism in these non-growing states affects the fermented product. Available studies on the response of L. cremoris to acid and starvation stress are based on population level data. We here characterized the energetic state and the protein synthesis capacity of stationary L. cremoris cultures at the single cell level. We show that glucose starved stationary cells are energy-depleted, while acid-induced stationary cells are energized and can maintain a pH gradient over their membrane. In the absence of glucose and arginine, a small pH gradient can still be maintained. Subpopulations of stationary cells can synthesize protein without a nitrogen source, and the subpopulation size decreases with increasing stationary phase length. Protein synthesis capacity during starvation only benefits culturability after 6 days. These results highlight significant differences between glucose starved stationary and acid-induced stationary cells. Furthermore, they show that the physiology of stationary phase L. cremoris cells is multi-facetted and heterogeneous, and the presence of an energy source during stationary phase impacts the cells capacity to adapt to their environment.
Highlights
Lactococcus cremoris [formerly Lactococcus lactis subsp. cremoris (Li et al, 2021)] is well-known for its use in food fermentations like cheese and yogurt (Kelleher et al, 2017; Pereira et al, 2020)
The results show that the fluorescence increase was not observed in the presence of the translation inhibitor, corroborating that the increase is caused by the production of new Green Fluorescent Protein (GFP) by the early-stationary cells (Figure 6A, bottom row)
Redon et al found the opposite for glucose starved L. lactis after 3.5 h, showing full retention of glycolytic enzyme activity (Redon et al, 2005)
Summary
Lactococcus cremoris [formerly Lactococcus lactis subsp. cremoris (Li et al, 2021)] is well-known for its use in food fermentations like cheese and yogurt (Kelleher et al, 2017; Pereira et al, 2020). Cremoris (Li et al, 2021)] is well-known for its use in food fermentations like cheese and yogurt (Kelleher et al, 2017; Pereira et al, 2020). During storage and ripening of these fermented products, L. cremoris spends weeks to months in a non-growing state. The cellular metabolism in this non-growing state can have a big impact on product properties, and there is great interest in understanding the physiology in nongrowing and stationary states of L. cremoris (van de Guchte et al, 2002; Papadimitriou et al, 2016; Nugroho et al, 2020). In a typical batch culture L. cremoris metabolizes sugars to lactic acid, thereby decreasing the pH until the sugars are depleted (glucose starved stationary) (Supplementary Information Section 1; Supplementary Figure 1).
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