CYTOCHROME-LINKED FERMENTATION IN BACTEROIDES RUMINICOLA

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White, D. C. (Rockefeller Institute, New York, N.Y.), M. P. Bryant, and D. R. Caldwell. Cytochrome-linked fermentation in Bacteroides ruminicola. J. Bacteriol. 84:822–828. 1962—Previous studies showed that Bacteroides ruminicola, an anaerobic, saccharolytic, ruminal bacterium, ferments glucose with the production of succinic, acetic, and formic acids, requires a large amount of CO2, and most strains require heme for growth. Difference spectra of cell suspensions of both heme-requiring strain 23, B. ruminicola subsp. ruminicola, and heme-independent strain GA33, B. ruminicola subsp. brevis, showed the presence of a cytochrome (absorption maxima at 560 mμ, near 530 mμ, and 428 mμ) similar to cytochrome b. This cytochrome and flavoprotein (trough at 450 mμ) in the cells, reduced by endogenous metabolism, were oxidized on addition of air, CO2, oxalacetate, malate, or fumarate but no oxidation occurred in the presence of succinate, malonate, lactate, pyruvate, aspartate, citrate, NO3−, SO4=, 2-n-heptyl or hydroxyquinoline-N-oxide (HOQNO), amytal or azide. The oxidation of these cellular pigments by fumarate was not inhibited by CN−, CO, malonate, succinate, amytal, or HOQNO. Glucose and reduced diphosphopyridine nucleotide (DPNH), but not succinate, reduced the pigments in frozen-thawed cells previously exposed to air for 4 hr at room temperature. The results suggest that this cytochrome and flavoprotein form an electron transport system for fumarate reduction to succinate by DPNH generated by glycolysis, and that succinate is produced via CO2 condensation with pyruvate or phosphoenolpyruvate and with oxalacetate, malate, and fumarate as intermediates. A pigment similar to cytochrome o (absorption maxima at 570, 555, and 416 mμ) was observed when reduced cells were treated with CO and compared to reduced cells, but there was no detectable cytochrome oxidase activity. The function of this pigment is obscure. No peroxidase or catalase activity was detected in either strain. Pyridine hemochromogens of both strains indicate one major heme, a protoheme-like pigment, with absorption in the α region maximum at 556 mμ. As B. ruminicola is one of the most numerous of rumen bacteria and ferments a wide variety of carbohydrates of importance in ruminant rations, cytochrome must be of importance in electron transport in rumen contents, a highly anaerobic environment.

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