# Alkaline pH‐Driven Metabolic Plasticity of Lactococcus lactis FM03

## TL;DR

Lactococcus lactis FM03 changes its metabolism and shape under alkaline conditions, which affects its growth and fermentation efficiency.

## Contribution

The study reveals how L. lactis adapts to alkaline pH through metabolic and morphological changes, offering new insights for industrial fermentation optimization.

## Key findings

- Alkaline pH causes a shift from mixed-acid to homolactic fermentation in L. lactis FM03.
- Proteome analysis shows downregulation of mixed-acid fermentation enzymes at alkaline pH.
- Alkaline pH leads to spherical cell morphology and reduced surface-to-volume ratio.

## 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.

Alkaline pH induces a metabolic shift in
Lactococcus lactis
FM03 from mixed‐acid to homolactic fermentation, driven by proteome‐level constraints in the mixed‐acid branch. This shift reduces biomass yield despite unchanged bioenergetics. Moreover, alkaline pH changes cell morphology and prevents elongation at higher growth rates. These adaptations offer insights into pH‐dependent resource allocation and opportunities for optimising industrial fermentations.