title: Effects of hops (Humulus lupulus L.) extract on volatile fatty acid production by rumen bacteria.
abstract: OBJECTIVE
  To determine the effects of hops extract on in vitro volatile fatty acid (VFA) production by bovine rumen micro-organisms.
  
  
  RESULTS
  When mixed rumen microbes were suspended in media containing carbohydrates, the initial rates of VFA production were suppressed by β-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, was decreased in the presence of hops extract.
  
  
  CONCLUSIONS
  These results are consistent with the hypothesis that the hops inhibit Gram-positive lactic acid bacteria (S. bovis), and the rumen microbial community requires a period of adaptation before normal VFA production resumes. Selenomonas 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: USDA, Agricultural Research Service, Forage-Animal Production Research Unit, Lexington, KY 40546, USA. michael.flythe@ars.usda.gov
authors.forename: M D
authors.initials: MD
authors.lastname: Flythe
authors.forename: G E
authors.initials: GE
authors.lastname: Aiken
mesh_terms: D000085:Acetates / Q000378:metabolism; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001419:Bacteria / Q000187:drug effects / Q000378:metabolism; D002417:Cattle; D018920:Coculture Techniques; D005232:Fatty Acids, Volatile / Q000378:metabolism*; D005285:Fermentation; D027582:Humulus; D010936:Plant Extracts / Q000494:pharmacology*; D011422:Propionates / Q000378:metabolism; D012417:Rumen / Q000382:microbiology*
chemical_list: D000085:Acetates; D000900:Anti-Bacterial Agents; D005232:Fatty Acids, Volatile; D010936:Plant Extracts; D011422:Propionates