In-vitro-Untersuchungen zur reduktiven Acetogenese im Pansen

The microbial pathway forming acetate from carbon dioxide and hydrogen is called reductive acetogenesis. This pathway occurs in the human colon as well as in the caecum of pigs and rabbits. Reductive acetogenesis is discussed as an alternative pathway to ruminal methanogenesis since reductive acetogene bacteria have been detected in the rumen. In the rumen acetogene bacteria form acetate by oxidation of easily soluble substances instead of the reduction of carbon dioxide with hydrogen. Starting with the difference between the microbial habitats of the caecum and rumen, the aim of the presented study was to investigate which factors may contribute to an initiation of reductive acetogenesis in two experimental steps. In the first step the influence of the factors presence of protozoa, diet, starving time of the donor animal and in-vitro-substrate were tested by means of the in-vitro-method batch culture. The results of these experiments were evaluated regarding their potential to initiate reductive acetogenesis by the criteria inhibition of methanogenesis, methane / SCFA (short chain fatty acids) ratio level and hydrogen recovery. Based on this evaluation, in the second step the effects of defaunation and endogenous substances of a sterilised caecum content were tested in batch culture. The influence of 0.4 and 0.6 % bile salt concentrations were screened in the RUSITEC-System. A procedure using the 13C-nuclear-mass-resonance spectroscopy (NMR) was employed to prove the presence of reductive acetogenesis directly. The results show that by eliminating the protozoa in the rumen the methanogenesis has been inhibited completely and the acetate / propionate ratio has been altered clearly in favour of the propionate. The 13C-CO2 marker procedure in the defaunted rumen liquid, did not reveal any indication of the occurrence of reductive acetogenesis. The influence of the factors diet, starving time of the donor animal and the in-vito- substrates hay and straw respectively did not point to a possible initiation of reductive acetogenesis by any of the measured fermentation parameters. The in-vitro-substrate sterilised caecum content and the addition of bile salts led to a significant inhibition of methanogenesis. Using the in-vitro-substrate sterilised caecum content, there was a reduced hydrogen recovery. The analysis with 13C-CO2- marker showed that the use of sterilised caecum content could not initiate the reductive analysis. In the rumen liquid, which was incubated in the RUSITEC-System for 7 days to examine the long term effect of bile salts, reductive acetogenesis could be detected in the control fermenter without bile salts and in the one with 0.4 % bile salt, but not in the fermenters with high bile salt concentration.