Abstract
The dairy starter Lactococcus lactis shifts its metabolism from mixed-acid fermentation to homolactic fermentation under anaerobic conditions as growth rates increase. Although its metabolism at acidic and neutral pH values is well-researched, knowledge about lactococcal physiology under alkaline conditions remains limited. Here, we investigated how L. lactis subsp. lactis biovar diacetylactis FM03 adapts its metabolism and morphology at alkaline pH using lactose-limited chemostat cultures at pH 6, 7 and 8. At alkaline pH, L. lactis FM03 shifted from energetically more favourable mixed-acid fermentation towards homolactic fermentation at lower growth rates compared to pH 6, resulting in a 20% lower biomass yield despite an unchanged maintenance coefficient and maximum biomass yield per ATP. Proteome analysis revealed a 1.5 to 13.5-fold downregulation of enzymes in the mixed-acid fermentation pathway at alkaline pH, thereby reducing its metabolic capacity. Morphologically, L. lactis became more spherical at alkaline pH, reducing the surface-to-volume ratio and did not enlarge upon higher dilution rates. This morphological shift potentially limits substrate uptake, contributing to the lower maximum growth rate at pH 8. Our findings reveal new insights into pH-driven metabolic plasticity and resource allocation in L. lactis and highlight opportunities for optimising fermentation processes under varying pH conditions.