Saponins containing methanol extract of Sapindus rarak affect microbial fermentation, microbial activity and microbial community structure in vitro

An in vitro fermentation was conducted to investigate the effect of saponins containing a methanol extract of Sapindus rarak (MS) on fermentation end products and microbial community structure and their activity with a view to ascertain their influence on rumen fermentation. MS was added to a final concentration of 0, 0.25, 0.5, 1.0, 2.0 and 4.0 mg/ml to an elephant grass and wheat bran (7:3 w/w) diet. An increase in short chain fatty acid (SCFA) production at 48 h of incubation occurred with increasing levels of MS, followed by a large shift in their molar proportions where acetate and butyrate decreased but propionate increased. Microbial biomass (mg/g substrate incubated) gradually increased with increasing MS from 102.8 (control) to 147.3 at 4 mg/ml MS (P < 0.10). A marked decrease in protozoal count was observed in the presence of MS. This was also in line with the result of membrane hybridization studies, which showed that no eukaryotic RNA could be detected in the presence of MS higher than 1 mg/ml. A higher bacterial RNA concentration was observed only at 1 mg/ml MS compared to control. Methanogen RNA concentration was lower at the highest concentration of MS (4 mg/ml). Fibrolytic microbes responded differently to MS. While no effect on Fibrobacter spp. was found, there was a negative effect on Ruminococcus albus and Ruminococcus flavefaciens at 2 and 4 mg/ml MS. Chytridiomycetes were reduced drastically at the highest level of MS. The xylanase activity and NDF digestibility decreased at higher levels of MS which may relate to the lower concentration of cell wall degrading bacteria. The study clearly marks the potential of S. rarak saponins as a de-faunating agent. However, a further evaluation of the effect on total tract cell wall degradation and on other beneficial effects of saponins when fed to animals is warranted.