Effect of steroidal saponin from Yucca schidigera extract on ruminal microbes

The effects of steroidal saponins (SAP) isolated from Yucca schidigera extr act on ruminal bacteria and fungi were investigated in pure culture studies . Prevotella bryantii, Ruminobacter amylophilus, Selenomonas ruminantium an d Streptococcus bovis were cultured through ten 24-h transfers in ruminal f luid medium containing 0 or 25 mu g SAP ml(-1) (measured as smilagenin equi valents). The four strains, each non-exposed or pre-exposed to SAP, were th en inoculated into medium containing 0 or 250 mu g smilagenin equivalents m l(-1) and 24-h growth curves were determined. The cellulolytic ruminal bact eria Ruminococcus flavefaciens, Fibrobacter succinogenes and Rc. albus were cultured for 72 h on Whatman no. 1 filter paper in medium containing 0, 9, 90 or 180 mu g SAP ml(-1) for the determination of filter paper digestion and endoglucanase activity. The ruminal bacteria differed in their response s to SAP. Steroidal saponins in the medium reduced the growth of Strep. bov is (P < 0.01 at 2, 3, 4, 5, 6 and 8 h), P. bryantii (P < 0.05 at 4, 5, 6, 8 , 10 and 24 h) and Rb. amylophilus (P < 0.05 at 14 and 24 h), but the growt h of S. ruminantium was enhanced (P < 0.05) at 10, 14 and 24 h. The growth curves of all four non-cellulolytic species were similar (P > 0.05) between pre-exposed and non-exposed cultures and the concentrations of total SAP a nd soluble (deglycosylated) SAP in the liquid fraction were unchanged (P > 0.05) over time. Steroidal saponins inhibited the digestion of filter paper by all three cellulolytic bacteria, but F. succinogenes was less (P < 0.05 ) sensitive to SAP and more (P < 0.05) effective at deglycosylating SAP tha n were Rc. flavefaciens or Rc. albus. Transmission electron microscopy reve aled that SAP altered the cell walls of the SAP-inhibited non-cellulolytic bacteria. The ruminal fungi, Neocallimastix frontalis and Piromyces rhizinf lata, were cultured on filter paper in medium containing 0, 0.45, 2.25 or 4 .5 mu g SAP ml(-1). Filter paper digestion by both fungi was completely inh ibited by 2.25 mu g SAP ml(-1). Steroidal saponins from Y. schidigera inhib it cellulolytic ruminal bacteria and fungi, but their effects on amylolytic bacteria are species dependent and similar to the effects of ionophores. A s such, SAP may be useful in nutritional applications targeting starch-dige sting ruminal microorganisms.