Submitted to: Antonie Van Leeuwenhoek
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/1996
Publication Date: N/A
Interpretive Summary: The microorganisms of the cow's digestive system are responsible for converting feed and forage materials into the fermentation acids used by the cow as a source of energy and milk production. The relative amounts of the different fermentation acids produced by these microbes vary with the type of microorganism and its growth environment, but the factors determining this variation have not been studied in detail. We studied acid formation by Ruminococcus flavefaciens FD-1, and important fiber digesting bacterium from the rumen, in order to determine how this bacterium controls acid formation. We found that when we varied the organism's growth rate and the acidity of its extracellular environment, the relative amounts of acetic and succinic acids (its major fermentation products) did not change significantly. However, as the organism grew faster, it produced more formic acid and less hydrogen gas. In the rumen, both of these products are wasteful since they are utilized by other microbes to form methane gas. It appears that formic acid and hydrogen may be produced because PEP carboxykinase, one of the enzymes involved in succinic acid production, may not be operating fast enough to use the metabolic intermediates of cellulose digestion. We have identified the means by which one of the most important bacteria responsible for digesting fiber in the rumen produces a high yield of useful fermentation end- products. The data suggest that optimizing the amount or activity of this bacterial species in the rumen will enhance the production of acetate, a necessary precursor of milkfat.
Technical Abstract: A pathway for conversion of the metabolic intermediate phosphoenolpyruvate (PEP) to acetate, succinate, formate, and hydrogen in ruminococcus flavefaciens FD-1 was constructed on the basis of enzyme activities detected in extracts of cells grown in cellulose- or cellobiose-limited continuous culture [dilution rate (D)=0.0017-0.244 h**-1, pH=6.02-7.08]. PEP was converted to acetate and CO2 (via pyruvate kinase, pyruvate dehydrogenase, and acetate kinase) or carboxylated to form succinate (via PEP carboxykinase, malate dehydrogenase, fumarase, and fumarate reductase). Lactate was not formed even during rapid growth (batch culture, mu=0.349 h**-1. H2 was formed by a hydrogenase rather than by cleavage of formate, and 13**C-NMR and 14**C-exchange reaction data indicated that formate was produced by CO2 reduction, not by a cleavage of pyruvate. The distribution of PEP into the acetate and succinate pathways was not affected by changing gextracellular pH and growth rates within the normal growth range. However increasing growth rate from 0.02 h**-1 to 0.244 h**-1 resulted in a shift toward formate production at the expense of H2. This shift suggested that reducing equivalents could be balanced through formate or hydrogen production without affecting the yields of the major carbon-containing fermentation end products.