Enrichment and Isolation of Rumen Bacteria That Reduce trans- Aconitic Acid to Tricarballylic Acid

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Bacteria from the bovine rumen capable of reducing trans-aconitate to tricarballylate were enriched in an anaerobic chemostat containing rumen fluid medium and aconitate. After 9 days at a dilution rate of 0.07 h−1, the medium was diluted and plated in an anaerobic glove box. Three types of isolates were obtained from the plates (a crescent-shaped organism, a pleomorphic rod, and a spiral-shaped organism), and all three produced tricarballylate in batch cultures that contained glucose and trans-aconitate. In glucose-limited chemostats (0.10 h−1), trans-aconitate reduction was associated with a decrease in the amount of reduced products formed from glucose. The crescent-shaped organism produced less propionate, the pleomorphic rod produced less ethanol, and the spiral made less succinate and possibly H2. Aconitate reduction by the pleomorphic rod and the spiral organism was associated with a significant increase in cellular dry matter. Experiments with stock cultures of predominant rumen bacteria indicated that Selenomonas ruminantium, a species taxonomically similar to the crescent-shaped isolate, was an active reducer of trans-aconitate. Strains of Bacteroides ruminicola, Butyrivibrio fibrisolvens, and Megasphaera elsdenii produced little if any tricarballylate. Wolinella succinogenes produced some tricarballylate. Based on its stability constant for magnesium (Keq = 115), tricarballylate could be a factor in the hypomagnesemia that leads to grass tetany.

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