Quantitative physiology of Lactococcus lactis at extreme low-growth rates

Summary This paper describes the metabolic adaptation of Lactococcus lactis during the transition from a growing to a non‐growing state using retentostat cultivation. Under retentostat cultivation, the specific growth rate decreased from 0.025 h−1 to 0.0001 h−1 in 42 days, while doubling time increased to more than 260 days. Viability of the overall culture was maintained above 90% but included approximately 20% damaged cells, which had lost their colony forming capacity on solid media. Although culture biomass and viability had reached a steady‐state after 14 days of retentostat cultivation, the morphology of the cells changed from coccus‐to‐rod shape at later stages of retentostat cultivation, by which the cell's surface to volume ratio was estimated to increase 2.4‐fold. Furthermore, the metabolic patterns switched between homolactic and mixed‐acid fermentation during the retentostat cultivation. Retentostat cultivation enabled the calculation of accurate substrate‐ and energy‐related maintenance coefficients and biomass yields under non‐growing conditions, which were in good agreement with those calculated by extrapolation from chemostat cultivations at high dilution rates. In this study, we illustrate how retentostat cultivation allows decoupling of growth and non‐growth associated processes in L. lactis, enabling the analysis of quantitative physiological responses of this bacterium to near zero‐specific growth rates.