Inhibition of fructan-fermenting equine fecal bacteria and Streptococcus bovis by hops (Humulus lupulus L.) ß-acid

Authors: HARLOW, BRITTANY - University Of Kentucky; LAWRENCE, LAURIE - University Of Kentucky; Kagan, Isabelle; Flythe, Michael

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2014
Publication Date: 4/29/2014
Citation: Harlow, B.E., Lawrence, L.M., Kagan, I., Flythe, M.D. 2014. Inhibition of fructan-fermenting equine fecal bacteria and Streptococcus bovis by hops (Humulus lupulus L.) ß-acid. Journal of Applied Microbiology. 117:329-339.

Interpretive Summary: The diets of modern horses are often rich in non-structural carbohydrates (e.g. simple sugars, starches and fructans). Excess carbohydrate has been associated with metabolic diseases such as hindgut acidosis and laminitis. In extreme cases, laminitis can lead to permanent damage to the hoof. The sources of nonstructural carbohydrates include grain, but most laminitis occurs when horses are kept on lush grass pastures that are rich in fructans. When fructans and other non-structural carbohydrates, in excess of what can be absorbed in the small intestine, are fermented by bacteria when they reach the hindgut. The bacteria convert the fructans to lactic acid the decrease the pH of the hindgut and produce toxins that lead to inflammation. Antibiotics have been used as a prophylactic and an intervention for pasture-associated laminitis (PAL). This study tests the idea that a plant-based antimicrobial can be used to stop the fructan-utilizing bacteria that cause PAL. The hops plant (Humulus lupulus) is well known for the ability to prevent the growth of certain lactic acid-producing bacteria. The experiments were performed with pure bacterial cultures as well as natural, uncultured bacteria taken from horses. The antimicrobial '-acid from hops destroyed the integrity of the cell membrane of the fructan-utilizing bacteria, which prevented their growth. Because the bacteria could not grow well in the presence of the hops '-acid, they could not make as much lactic acid and pH stayed relatively neutral. These results show that plant-based additives, like hops, could be used in place of antibiotics to address the problem of PAL. Hops has been consumed by humans for centuries, but the safety and efficacy for horses will have to be determined before giving hops to horses could be recommended.

Technical Abstract: Aims: The goals were to determine if the '-acid from hops (Humulus lupulus L.) could be used to control fructan fermentation by equine hindgut microorganisms, and to verify the antimicrobial mode of action on the Streptococcus bovis, which has been implicated in fructan fermentation, hindgut acidosis, and pasture associated laminitis (PAL) in the horse. Methods and Results: Suspensions of uncultivated equine fecal microorganisms produced short chain fatty acids when fructan (i.e. inulin) was the substrate, but '-acid (i.e. lupulone) concentrations = 9 ppm inhibited lactic acid production and mitigated the decrease in pH. Inulin-fermenting S. bovis was isolated from the suspensions. All of the isolates were sensitive to '-acid, which decreased viable number of streptococci in fecal suspensions, as well as the growth, lactate production, and the intracellular potassium of S. bovis in pure culture. Conclusions: These results are consistent with the hypothesis the hops '-acid prevented acidification by fructan-fermenting equine fecal bacteria by dissipating the transmembrane ionic gradients of S. bovis and other Gram-positive lactic acid bacteria and thereby preventing their growth and lactic acid production. Significance and Impact of the Study: Hindgut fermentation of grass fructans has been linked to PAL and other metabolic disorders in horses. Hops '-acid is a potential phytochemical intervention.