Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/27/1998
Publication Date: N/A
Citation: Interpretive Summary: Ruminant animals (e.g., dairy cows) use bacteria to break down plants so that the animal can gain nutritive benefit from their feeds. Non-ruminants, such as ourselves, cannot digest much of a plant fiber. Ruminants produce methane and hydrogen during digestion of their feed. The process called methanogenesis causes digestible energy (in the form of methane) to be lost tby ruminants. However, this process is important because it is a means for removing the hydrogen from the ruminal environment -- accumulation of hydrogen suppresses digestion. Our strategy is to re-direct hydrogen disposal by replacing methanogenesis with a different process called "reductive acetogenesis". The latter process consumes hydrogen and produces acetic acid, a usable form of energy by ruminants, instead of methane. We determined the populations of "acetogenic" bacteria in the rumens of beef cows fed hay and steers fed a diet high in grain to prepare them for slaughter. Small populations were found in both situations, but larger populations were found in beef cows. Subsequent experimentation revealed that reductive acetogenesis could become an important hydrogen-consuming process if (1) methanogenesis was selectively suppressed and (2) the ruminal acetogenic population was increased. These studies are eventually aimed at improving plant utilization by cattle, minimizing wastes, and improving sustainability of dairy operations.
Technical Abstract: The objective of this study was to evaluate the role of reductive acetogenesis as an alternative H2 disposal mechanism in the rumen. H2/CO2- supported acetogenic bacteria were enumerated from the rumen using a selective inhibitor of methanogenesis, 2 bromoethanesulfonic acid (BES). Acetogenic bacteria ranged in density from 2.5 x 10**5 cells/ml in beef cows fed a high forage diet to 75 cells/ml in finishing steers fed a high grain diet. Negligible endogenous acetogenic activity was demonstrated in incubations containing ruminal contents, NaH**13CO3, and 100% H2 gas phase since [U-**13C]-acetate, as measured by mass spectroscopy, did not accumulate. Enhancement of acetogenesis was observed in these incubations when methanogenesis was inhibited by BES and/or by the addition of an axenic culture of the rumen acetogen Acetitomaculum ruminis 190A4 (10**7 CFU/ml). To assess the relative importance of population density and/or H2 concentration for reductive acetogenesis in ruminal contents, incubations as described above were formed under a 100% N2 gas phase. Both selective inhibition of methanogenesis and A. ruminis 190A4 fortification (>10**5 CFU/ml) were necessary for the detection of reductive acetogenesis under H2 limiting conditions. Under these conditions H2 accumulated to 4800 ppm. In contrast, H2 accumulated to 400 ppm in incubations with active methanogenesis (without BES). The H2 concentrations correlated well with the pure culture H2 threshold concentrations determined for A. ruminis 190A4 (3830 ppm) and a ruminal methanogen 10-16B (126 ppm), respectively. The data demonstrate that ruminal methanogenic bacteria limited reductive acetogenesis by lowering the H2 partial pressure below the level necessary for H2 utilization by A. ruminis 190A4.