In this study, we used chemostat cultures to analyze the quantitative effects of the specific growth rate and respiration on the metabolism in L. lactis CHCC2862, and on the downstream robustness of cells after freezing or freeze-drying. Under anaerobic conditions, metabolism remained homofermentative, although biomass yields varied with the dilution rate (D). In contrast, metabolism shifted with the dilution rate under respiration-permissive conditions. At D = 0.1 h^-1, no lactate was produced, while lactate formation increased with higher dilution rates. Thus, a clear metabolic shift was observed, from flavor-forming respiratory metabolism at low specific growth rate, to mixed-acid respiro-fermentative metabolism at higher specific growth rates. Quantitative analysis of the respiratory activity, lactose uptake rate and metabolite production rates showed that aerobic acetoin formation provided most of the NADH consumed in respiration. Moreover, the maintenance-associated lactose consumption under respiration-permissive conditions was only 10 % of the anaerobic value, either due to higher respiratory yield of ATP on consumed lactose or due to lower maintenance-related ATP demand. The cultivation conditions also affected the quality of the starter cultures produced. Cells harvested under respiration-permissive conditions at D = 0.1 h^-1 were less robust after freeze-drying and had a lower acidification activity for subsequent milk acidification, whereas respiration-permissive conditions at the higher dilution rates led to robust cells that performed equally well or better than anaerobic cells.

Importance Lactococcus lactis is used in large quantities by the food and biotechnological industries. L. lactis can use oxygen for respiration if heme is supplied in the growth medium. This has been extensively studied in batch cultures using various mutants, but quantitative studies of how the cell growth affects respiratory metabolism, energetics and cell quality are surprisingly scarce. Our results demonstrate that the respiratory metabolism of L. lactis is remarkably flexible and can be modulated by controlling the specific growth rate. We also link the physiological state of cells during cultivation to the quality of frozen or freeze-dried cells, which is relevant to the industry that may lack understanding of such relationships. This study extends our knowledge on respiratory metabolism in L. lactis and its impact on frozen and freeze-dried starter culture products, and it illustrates the influence of cultivation conditions and microbial physiology on the quality of starter cultures.

在这项研究中，我们使用了恒化器培养物来分析特定生长速率和呼吸作用对乳酸乳球菌CHCC2862的代谢以及冷冻或冻干后细胞下游健壮性的定量影响。在厌氧条件下，尽管生物质产量随稀释率（D）而变化，但新陈代谢仍保持同型发酵。相反，在允许呼吸的条件下，新陈代谢随稀释率而变化。在D = 0.1 h ^-1时，没有产生乳酸，而随着稀释率的升高，乳酸的形成增加。因此，观察到明显的新陈代谢转变，从低比生长速率下形成风味的呼吸代谢到高比生长速率下的混合酸呼吸发酵代谢。对呼吸活动，乳糖摄取率和代谢产物产生率的定量分析表明，有氧乙酰丙酮的形成提供了呼吸中消耗的大部分NADH。此外，在呼吸允许的条件下，与维持相关的乳糖消耗仅为厌氧值的10％，这是由于消耗的乳糖对ATP的呼吸产量更高或与维持相关的ATP需求较低所致。培养条件也影响所产生的发酵剂培养的质量。D = 0.1 h ^-1冻干后的健壮性较低，并且对于随后的牛奶酸化具有较低的酸化活性，而在较高稀释率下的呼吸允许条件导致健壮的细胞表现得比厌氧细胞好或更好。

重要性 食品和生物技术行业大量使用乳酸乳球菌。如果在生长培养基中提供血红素，则乳酸乳球菌可以使用氧气进行呼吸。在使用各种突变体的分批培养中已经对此进行了广泛的研究，但是令人惊讶地缺乏关于细胞生长如何影响呼吸代谢，能量和细胞质量的定量研究。我们的结果证明乳酸乳球菌的呼吸代谢是非常灵活的，可以通过控制特定的增长率进行调节。我们还将培养过程中细胞的生理状态与冷冻或冻干细胞的质量联系起来，这与可能缺乏对这种关系的了解的行业有关。这项研究扩展了我们对乳酸乳球菌呼吸代谢及其对冷冻和冻干发酵剂发酵产物的影响的知识，并阐明了培养条件和微生物生理学对发酵剂发酵质量的影响。