Ability of Acidaminococcus fermentans to oxidize trans-aconitate and decrease the accumulation of tricarballylate, a toxic end product of ruminal fermentation

Abstract

Mixed ruminal bacteria convert trans-aconitate to tricarballylate, a tricarboxylic acid which chelates blood divalent cations and decreases their availability (J. B. Russell and P. J. Van Soest, Appl. Environ. Microbiol. 47:155-159, 1984). Decreases in blood magnesium in turn cause a potentially fatal disease known as grass tetany. trans-Aconitate was stoichiometrically reduced to tricarballylate by Selenomonas ruminantium, a common ruminal bacterium in grass-fed ruminants (J. B. Russell, Appl. Environ. Microbiol. 49:120-126, 1985). When mixed ruminal bacteria were enriched with trans- aconitate, a trans-aconitate-oxidizing bacterium was also isolated (G. M. Cook, F. A. Rainey, G. Chen, E. Stackebrandt, and J. B. Russell, Int. J. Syst. Bacteriol. 44:576-578, 1994). The trans-aconitate-oxidizing bacterium was identified as Acidaminococcus fermentans, and it converted trans- aconitate to acetate, a nontoxic end product of ruminal fermentation. When S. ruminantium and A. fermentans were cocultured with trans-aconitate and glucose, tricarballylate never accumulated and all the trans-aconitate was converted to acetate. Continuous-culture studies (dilution rate, 0.1 h-1) likewise indicated that A. fermentans could outcompete S. ruminantium for trans-aconitate. When mixed ruminal bacteria were incubated in vitro with 10 mM trans-aconitate for 24 h, 45% of the trans-aconitate was converted to tricarballylate. Tricarballylate production decreased 50% if even small amounts of A. fermentans were added to the incubation mixes (0.01 mg of protein per mg of mixed bacterial protein). When A. fermentans (2 g of bacterial protein) was added directly to the rumen, the subsequent conversion of trans-aconitate to tricarballylate decreased 50%, but this effect did not persist for more than 18 h. A. fermentans eventually washed out of the rumen because the grass hay diet fed to the cow did not contain energy sources which could be utilized by A. fermentans. Because of its rapid rate of trans- aconitate oxidation and ability to compete with other ruminal bacteria for trans-aconitate, A. fermentans may have potential for preventing the sudden increases in tricarballylate levels which are associated with grass tetany.