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.

Resource allocation explains lactic acid production in mixed‐culture anaerobic fermentations

“…This result suggests that auxotrophic LAB outcompete BPB in environments that select for microorganisms with the highest maximum specific growth rate, as, for example, in sequencing batch reactors. The estimated kinetic parameters of the two microbial communities in the experiments of Rombouts et al (2020) also show that LAB in rich medium also developed a higher maximum growth rate (0.23 h −1 ) than BPB in mineral medium. (0.19 h −1 ), which represent a 20% increase.…”

“…Information on this subject for LAB or other prokaryotic microorganisms was not available in literature. Energy requirements for transport of substrates (glucose, inorganic nitrogen, or amino acids) and end‐products is not considered as it depends highly on the intracellular concentrations of the substrates and products to be transported, which are not simulated in the model. This assumption does not affect the competition between auxotrophic LAB and BPB since the energy required for transporting amino acids or inorganic nitrogen is similar (Rombouts et al, 2020; Stouthamer, 1973). …”

“…Recently, it was reported that lactate can dominate the product spectrum in MCF dominated by species from the genus Lactobacillus and Lactococcus , but only under very specific operational conditions (Rombouts et al, 2020). A complex medium with peptides and vitamins is needed to grow lactic acid bacteria (LAB) because they are auxotrophic for amino acids and some vitamins meaning that they are not capable of producing these compounds from the primary carbon substrate (Cocaign‐Bousquet et al, 1995; Magnúsdóttir et al, 2015; Wang et al, 2015).…”

“…A complex medium with peptides and vitamins is needed to grow lactic acid bacteria (LAB) because they are auxotrophic for amino acids and some vitamins meaning that they are not capable of producing these compounds from the primary carbon substrate (Cocaign‐Bousquet et al, 1995; Magnúsdóttir et al, 2015; Wang et al, 2015). Rombouts et al (2020) performed glucose MCFs experiments in a sequential batch reactor and observed that when using a rich medium containing peptides and B vitamins the community was dominated by LAB ( Lactococcus and Lactobacillus species) but when the employing a mineral medium glucose was fermented to mainly acetate and butyrate by an Ethanoligenens species from the Clostridia class.…”

“…Also environmental conditions that select for maximum specific growth rate, as in batch processes, are a prerequisite for LAB to outcompete other bacterial groups, as shown in (Rombouts et al, 2020). Fermentations carried out under other conditions lead inevitably to the production of a mixture of other carboxylic acids and are dominated by species of the genus Clostridium or Ethanololigenes (Rombouts et al, 2019, 2020; Temudo et al, 2007).…”