Selecting for lactic acid producing and utilising bacteria in anaerobic enrichment cultures

Abstract

Lactic acid‐producing bacteria are important in many fermentations, such as the production of biobased plastics. Insight in the competitive advantage of lactic acid bacteria over other fermentative bacteria in a mixed culture enables ecology‐based process design and can aid the development of sustainable and energy‐efficient bioprocesses. Here we demonstrate the enrichment of lactic acid bacteria in a controlled sequencing batch bioreactor environment using a glucose‐based medium supplemented with peptides and B vitamins. A mineral medium enrichment operated in parallel was dominated by Ethanoligenens species and fermented glucose to acetate, butyrate and hydrogen. The complex medium enrichment was populated by Lactococcus, Lactobacillus and Megasphaera species and showed a product spectrum of acetate, ethanol, propionate, butyrate and valerate. An intermediate peak of lactate was observed, showing the simultaneous production and consumption of lactate, which is of concern for lactic acid production purposes. This study underlines that the competitive advantage for lactic acid‐producing bacteria primarily lies in their ability to attain a high biomass specific uptake rate of glucose, which was two times higher for the complex medium enrichment when compared to the mineral medium enrichment. The competitive advantage of lactic acid production in rich media can be explained using a resource allocation theory for microbial growth processes.

Abstract

The supplementation of peptides and B vitamins to an ecosystem where glucose was fermented directs the glucose to lactate and ethanol. Enriching with glucose and a mineral medium, acetate, butyrate and hydrogen was the dominant product pathway. This is likely caused by proteome optimisation of lactic acid bacteria. Also, lactate was only fermented in the presence of B vitamins and peptides. We demonstrate the rate efficiency trade off in fermentative ecosystems and pave the way for directing product formation.