This study demonstrates that Veillonella parvula, an anaerobic bacterium, cannot grow on succinate alone but uses succinate decarboxylation to enhance growth yields when fermenting other substrates. When succinate was provided alongside lactate or malate, growth yields increased by 2.4–3.5 g per mole of succinate consumed. During lactate fermentation, the bacterium produced acetate, propionate, and hydrogen gas, while malate fermentation yielded only acetate and propionate. Aspartate utilization required succinate as a co-substrate. Enzyme analysis revealed that V. parvula uses methylmalonyl-CoA decarboxylase and pyruvate carboxylase—rather than the transcarboxylase enzyme found in classical propionic acid bacteria—to metabolize these substrates. The decarboxylation of methylmalonyl-CoA creates a sodium ion gradient across the cell membrane, which apparently helps conserve metabolic energy for processes like substrate uptake, even though the bacterium lacks the sodium-dependent ATPase needed to directly harness this gradient for growth on succinate alone.

Key findings

• Succinate decarboxylation by V. parvula increases growth yields on lactate or malate by 2.4–3.5 g mol⁻¹, despite the bacterium being unable to grow on succinate as the sole energy source.
• V. parvula uses methylmalonyl-CoA decarboxylase and pyruvate carboxylase rather than transcarboxylase for C₄-dicarboxylic acid metabolism, distinguishing it from classical Propionibacterium species.
• Methylmalonyl-CoA decarboxylase generates a Na⁺ gradient across the cell membrane that appears to facilitate aspartate uptake and energy conservation for other metabolic processes.
• Lactate fermentation produces acetate, propionate, and H₂, while malate fermentation yields acetate and propionate without H₂ production.