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.