Propionibacterium freudenreichii thrives in microaerobic conditions by complete oxidation of lactate to CO₂

In this study we show increased biomass formation for four species of food-grade propionic acid bacteria (Acidipropionibacterium acidipropionici, Acidipropionibacterium jensenii, Acidipropionibacterium thoenii and Propionibacterium freudenreichii) when exposed to oxygen, implicating functional respiratory systems. Using an optimal microaerobic condition, P. freudenreichii DSM 20271 consumed lactate to produce propionate and acetate initially. When lactate was depleted propionate was oxidized to acetate. We propose to name the switch from propionate production to consumption in microaerobic conditions the ‘propionate switch’. When propionate was depleted the ‘acetate switch’ occurred, resulting in complete consumption of acetate. Both growth rate on lactate (0.100 versus 0.078 h⁻¹) and biomass yield (20.5 versus 8.6 g* mol⁻¹ lactate) increased compared to anaerobic conditions. Proteome analysis revealed that the abundance of proteins involved in the aerobic and anaerobic electron transport chains and major metabolic pathways did not significantly differ between anaerobic and microaerobic conditions. This implicates that P. freudenreichii is prepared for utilizing O₂ when it comes available in anaerobic conditions. The ecological niche of propionic acid bacteria can conceivably be extended to environments with oxygen gradients from oxic to anoxic, so-called microoxic environments, as found in the rumen, gut and soils, where they can thrive by utilizing low concentrations of oxygen.