Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2010
Publication Date: 4/1/2010
Citation: Flythe, M.D., Aiken, G.E. 2010. Effects of hops (Humulus lupulus L.) extract on volatile fatty acid production by rumen bacteria. Journal of Applied Microbiology. 109(4):1169-1176. Interpretive Summary: Ruminants are unique in that the first stage of digestion is carried out by symbiotic microbes that colonize the rumen. When a ruminant feeds, bacteria ferment the plant tissues, and generate many products, including volatile fatty acids (VFA). The VFA are absorbed directly through the rumen epithelium, transported into the blood stream and undergo gluconeogenesis. In this way, the animal is rapidly provided with blood sugar, and the microbes are freed from end-product inhibition. A principle theory in rumen microbiology is that the activities of the microbes can be optimized just like any other fermentation. This concept has been applied in a number of ways, and antimicrobial feed additives are one of the most successful. The hops beta-acids (plant secondary metabolites including: lupulone, colupulone, and adlupulone) had a positive effect on VFA production by rumen bacteria. When hops extract was added to in vitro fermentations, a greater proportion of propionic acid was produced. This latter VFA is a nutritionally important gluconeogenesis precursor in ruminants. There was an initial lag in VFA production when hops extract was added. Experiments with pure cultures of rumen bacteria reviled that the lag could be explained by the differing sensitivities of common cell envelope types. These results are similar to those achieved with ionophores, which have been used as antimicrobial growth promoters for several decades.
Technical Abstract: Aims: To determine the effects of hops extract, on in vitro volatile fatty acid (VFA) production by bovine rumen microorganisms. Methods and Results: When mixed rumen microbes were suspended in media containing carbohydrates, the initial rates of VFA production were suppressed by beta-acid rich hops extract. The rates of VFA production increased over extended incubations (24 h), and hops extract caused an increase in the propionate to acetate ratio. Hops extract inhibited the growth and metabolism of Streptococcus bovis, but Selenomonas ruminantium and Megasphaera elsdenii were not affected. Likewise, the propionate production of M. elsdenii/S. bovis co-cultures, but not M. elsdenii/S. ruminantium co-cultures were decreased in the presence of hops extract. Conclusions: These results are consistent with the hypothesis that the hops inhibits Gram-positive lactic acid bacteria (S. bovis), and the rumen microbial community requires a period of adaptation before normal VFA production resumes. S. bovis and S. ruminantium both produce lactate, which is the substrate for propionate production by M. elsdenii. However, S. ruminantium has an outer membrane, while S. bovis does not. Significance and Impact of Study: The enhanced production of the gluconeogenesis precursor, propionic acid, provides further evidence that plant secondary metabolites from hops could be used to improve rumen fermentation.