Bioenergetics of complex and mineral-type fermentation: supplementation might lead to a more efficient metabolism
The lower growth yield in the complex medium (Table 1) is counterintuitive since these bacteria grow on a complex medium and do not need to produce the amino acids themselves. To compare the impact of the supplementation of peptides and B vitamins on the metabolism, we calculated the Yx,ATP for both enrichments from the catabolic ATP yields estimated (Table 1). For the mineral enrichment a 11% lower Yx,ATP value was estimated, but is not statistically significant (one-tailed t-test gives p=0.45).. Amino acid degradation (Stickland fermentation) was omitted from this bioenergetic evaluation, as the metabolic evaluation of 20 different amino acid utilising pathways would seriously complicate the bioenergetic evaluation. Incorporating amino acid degradation might lower the YX,ATP for the complex medium enrichment, bringing it closer to the value of the mineral medium enrichment.
Prototrophic fermenters such as Escherichia coli and auxotrophic fermenters such as Lactococcus lactis have similar protein and RNA content (Table S6). Stouthamer estimated that the supplementation of amino acids induces a 0.7% decrease in Yx,ATP­ (Stouthamer, 1973). The biomass yield of E. coli fermenting glucose in complex medium is 13% higher than when fermenting in mineral medium (Lawford and Rousseau, 1995). According to Stouthamer (1973, Table 5) the synthesis of amino acids consumes a relatively low amount of ATP, while uptake of amino acids or ammonium consumes ATP, making both environment bioenergetically equivalent. The polymerisation process for proteins consumes most ATP, about 55% of the available ATP (Stouthamer, 1973). We expect that the biosynthesis of B vitamins requires a relatively small ATP-flux, as B vitamins are present in trace amounts in bacterial biomass (<10-5 in g g-1) (Waller and Lichstein, 1965).
The difference in anabolic efficiency we estimated is much less than the difference observed in µmax, indicating that the complex medium promotes high substrate uptake rates rather than high biomass yields. Functional protein is a valuable resource for a cell. Not only minimising the fermentative pathway length, but also minimising biosynthetic enzymes for amino acid and B vitamin production makes that more cellular protein can be allocated to increase the growth rate of the cell. This is in essence the “trick” that lactic acid bacteria use to dominate anaerobic environments where carbohydrates and peptides are available.