Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 8, 2005
Publication Date: February 26, 2006
Citation: Kronberg, S.L., Halaweish, F.T., Hubert, M.B., Weimer, P.J. 2006. Interactions between euphorbia esula toxins and bovine ruminal microbes. Journal of Chemical Ecology. 32(1):15-28. Interpretive Summary: It is well known that cattle graze little if any of the noxious weed leafy spurge while sheep and goats will usually ingest considerable amounts of it once they adapt to consuming it. Previous research has indicated that a critical difference between cattle and these small ruminants in respect to their avoidance or ingestion of leafy spurge may involve differences in how the microbes in their rumens interact with toxic compounds in the weed. This study evaluated how the toxicity of leafy spurge changed over time when it was fermented with ruminal microbes from cattle and sheep. It also evaluated how the toxicity of leafy spurge changed over time when it was fermented with ruminal microbes from cattle that were exposed to a gram-negative antibiotic, and how the growth of several species of ruminal bacteria responded to toxins from leafy spurge. Lastly, the study determined the levels of a leafy spurge toxin (ingenol) in fermented and non-fermented mixtures of leafy spurge and cattle ruminal microbes. In general, toxicity of leafy spurge that was fermented with cattle or goat ruminal microbes increased with some fermentation time and there were few differences in toxicity as a function of the source of ruminal microbes (cattle versus goats). Leafy spurge toxins had little negative impact on growth of several species of ruminal bacteria. Levels of the leafy spurge toxin ingenol appeared to decrease with fermentation even though the toxicity of the fermented mixtures tended to increase. This indicated that ruminal microbes probably converted ingenol into one or more other toxic compounds.
Technical Abstract: Toxicity of leafy spurge (LS; Euphorbia esula L.) compounds to brine shrimp after in vitro fermentation with cattle versus goat rumen digesta or with normal versus modified cattle rumen digesta was evaluated at different lengths of fermentation. Levels of the toxin ingenol were determined for fermented and non-fermented mixtures of LS and cattle rumen digesta, and the toxicity of an extract of LS to common rumen microbes was evaluated. Length of fermentation of LS with cattle versus goat rumen digesta affected the toxicity of these mixtures to brine shrimp differently. In general, toxicity of the mixtures of LS and either cattle or goat digesta was greater after some fermentation time compared to no fermentation. The only combination that resulted in significant shrimp toxicity without fermentation was with the highest extract concentration of LS and cattle digesta. This result offered a potential explanation for why cattle learn to avoid LS while goats learn to ingest it. Fermentation of LS with cattle digesta in the second trial also increased the toxicity of extracted compounds to brine shrimp. Introduction of an antibiotic that preferentially inhibits gram-negative bacteria into the LS and cattle rumen digesta mixtures did not appear to affect their toxicities regardless of fermentation length. Levels of toxic ingenol were observed in LS and cattle digesta mixtures (both fermented and non-fermented) that were consistent with the levels of ingenols reported for LS. Finally, a toxic extract of LS had little or no negative effect on the growth of several common species of rumen bacteria. Key Words'Euphorbia esula, leafy spurge, ingenol, ruminal microbes, fermentation, cattle, goats.