Supplementation of peptides and B vitamins leads to dominance of lactic acid bacteria and high qsmax through resource allocation
In this work we demonstrate that lactic acid bacteria outcompete prototrophic type fermenters (e.g. , Clostridium species) when nutritive conditions were favourable, i.e. , with sufficient amount of amino acids and B vitamins in an SBR cultivation mode. Kim and colleagues (Kim et al., 2016) have shown that lactic acid bacteria can be enriched in a continuous-flow stirred tank reactor (CSTR) process. They operated the CSTR anaerobically, at pH 5.0 and thermophilic (50 °C) conditions with a SRT of 12 h, with glucose and yeast extract as fermentable organic substrates. Yeast extract is a well-known source of peptides, amino acids, B vitamins and carbohydrates. In cabbage fermentations lactic acid bacteria are known to be the dominant organism (Plengvidhya et al., 2007), while fermentable substrates with low protein content, such as starch, Clostridium species are the dominant organism (Lin et al., 2008).
Lactic acid bacteria are well known to be auxotrophic for amino acids (Kitay and Snell, 1950), while their auxotrophy for B vitamins is more ambiguous. Some lactic acid bacteria might actually be producers of B vitamins (LeBlanc et al., 2011). Studies with lactic acid bacteria on synthetic medium have demonstrated the specific compounds needed for growth (Novak et al., 1997), up to individual amino acids (Cocaign-Bousquet et al., 1995). The effect of decreasing medium complexity has been illustrated by Olmos-Dichara et.al(Olmos-Dichara et al., 1997). When the “richness” of the growth medium was decreased, the qsmax remained stable, while the growth yield decreased. This shows that the medium complexity directly influences the bioenergetics of L. casei , resulting in a lower biomass production when peptides and/or B vitamins are insufficiently supplied in the medium.
Lactic acid bacteria have a kinetic advantage leading to their dominance in enrichments at high growth rates and complex media. The biomass yield of the complex medium enrichment culture was 20% lower than for the enrichment culture on a mineral medium. The maximal substrate uptake rate was almost double for the community enriched on a complex medium versus mineral medium (Table 1). Lactic acid production is clearly a metabolic strategy of high flux but low efficiency. This is supported by the observation that lactic acid bacteria switch to acetate and ethanol production when substrate conversion rates decrease, i.e. lower growth rates (De Vries et al., 1970). Acetate/ethanol production generates 3 instead of 2 moles ATP for lactate fermentation on glucose. This can be placed well in the context of resource allocation theories, given a certain protein budget (Bachmann et al., 2017). Less biosynthetic enzymes needed for amino acids and B vitamin synthesis lead to a smaller anabolic proteome. A smaller anabolic proteome can imply a bigger catabolic proteome, as demonstrated when comparing the proteome from E.coli grown in a mineral and complex medium (Li et al., 2014). Lactate catabolism requires one enzyme from pyruvate, while acetate/ethanol production requires at least 5 enzymes. Furthermore, at increasing growth rates, ribosome and RNA polymerase content is higher (Bosdriesz et al., 2015). Lactic acid bacteria are assumed to have optimally distributed their metabolic enzyme levels (Teusink et al., 2011), enabling a high overall metabolic flux.