Studies on anaerobic bacteria: an investigation of the growth and metabolism of the rumen bacterium bacteroides ruminicola

Abstract

Bacteroides ruminicola is known to have a high molar growth yield when grown on glucose, and this investigation aimed to seek an explanation for this. The acidic end-products of glucose fermentation by the bacterium are succinate, acetate and formate. Data from labeling experiments in which [U-14C] glucose, [1-14C] glucose or 14co2 were used, were consistent with a major contribution by the Embden-Meyerhof pathway to glucose fermentation, some 68% of succinate being formed from glucose via a carboxylation of phosphoenolpyruvate or pyruvate. When labeled peptides and acetate (initially present in the growth medium) were tested as possible non-glucose precursors of succinate, more than 23% of this product was formed from their metabolism. The electron transport system catalysing NADH oxidation by fumarate (which may or may not be linked to phosphorylation), has been studied using membrane preparations of B. ruminicola. Extraction and reincorporation studies showed that menaquinone is an essential electron transport component in the reaction. Evidence that the b-type cytochrome present is probably not on the main electron transport sequence(s) between NADH and fumarate in membrane preparations was obtained by comparing NADH: fumarate oxido-reductase activity with the rate of the b-type cytochrome reduction by NADH in the presence and absence of HOQNO. The results are analogous to those reported in non-phosphorylating submitochondrial preparations (Keilin-Hartree particles) where cytochrome b is displaced from the main pathway. Preliminary attempts to demonstrate electron transport linked phosphorylation during fumarate reduction in membrane preparations were not successful. Continuous culture studies support previous work (Howlett, 1974) that B. ruminicola has a high molar growth yield when growing on glucose. Within the range of dilution rates measured, the growth yield range was 56 - 57 g dry weight (corrected for storage polysaccharide) per mol of glucose fermented. Corrections were made for storage polysaccharide so that growth parameters could be calculated for cells of constant composition. Estimates of YATP were calculated several ways on the basis of several different assumptions. It was assumed that ATP was made (i) by substrate level phosphorylation alone, or by substrate level phosphorylation plus electron transport linked phosphorylation assuming (ii) a P/2e ratio of 1, or (iii) a P/2e ratio of 2. For each of the above three sets of calculations, a minimum and a maximum value of YATP was calculated assuming either, (a) that the ATP yield for the formation of each end-product was the same no matter what the precursor of its carbon skeleton, or (b) that there was no energy production by substrate level phosphorylation from the catabolism of non-glucose precursors. A comparison of a theoretical value for YmaxATP with each YmaxATP value (calculated on the above bases) obtained from reciprocal plots of YobsATP against dilution rate was made. It was concluded that YmaxATP estimates were nearest to the theoretical value when based on ATP yields assuming a P/2e range of 0 to 1 including non-integer values between. The study suggested a P/2e ratio of not more than one, or perhaps zero may be the correct value.