Effect of starch source (corn, oats or wheat) and concentration on fermentation by equine fecal microbiota in vitro

Authors: HARLOW, BRITTANY - University Of Kentucky; Donley, Taylor; LAWRENCE, LAURIE - University Of Kentucky; Flythe, Michael

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2015
Publication Date: 9/28/2015
Citation: Harlow, B.E., Donley, T.M., Lawrence, L.M., Flythe, M.D. 2015. Effect of starch source (corn, oats or wheat) and concentration on fermentation by equine fecal microbiota in vitro. Journal of Applied Microbiology. doi: 10.1111/jam.12927.

Interpretive Summary: Horses are often fed diets that include cereal grains to meet their energy requirements. The main energy source in cereal grains is starch. Starch is enzymatically digested and absorbed in the equine small intestine, but when there is a high-proportion of cereal grains in the diet, starch can reach the hindgut (cecum, large intestine). Starch-utilizing bacteria in the hindgut convert the starch to lactic acid, which can acidify the hindgut of the horse. Hindgut acidosis, whether acute or chronic, is a serious health issue for horses. It has been reported that some grain types, such as oats, are more completely digested in the small intestine so that less starch is delivered to the hindgut. This study was initiated to determine if equine hindgut bacteria also utilize corn, wheat and oats differently. Uncultivated bacteria were non-invasively harvested from horse feces and prepared in microbial cell suspensions. The cell suspensions were incubated with ground corn, wheat or oats (balanced by starch concentration). The fermentation products (acids and gas) and the number of specific groups of starch-utilizing bacteria were monitored during 24-hour incubations. The cell suspensions made more gas from oats and wheat than corn, but corn led to more lactic acid production and lower pH values. The predominant starch-utilizing bacteria isolated from oat fermentations was Streptococcus bovis, but Enterococcus faecalis was dominant in corn and wheat fermentations. Both of these bacteria are well-known lactic acid producers. The conclusion was that horse gut microorganisms, even apart from digestion by the equine host, use corn, wheat and oats differently. S. bovis and E. faecalis were identified as possible culprits in the excessive conversion of starch to lactic acid, and they will be the focus of a grain feeding trial that follows this laboratory study. The long-term impact is to provide management strategies, such as diet design and microbial interventions, to reduce hindgut acidosis and related problems.

Technical Abstract: Aims: The goal was to determine the effect of starch source (corn, oats and wheat) and concentration on: 1) total amylolytic bacteria, Group D Gram-positive cocci (GPC), lactobacilli, and lactate-utilizing bacteria, and 2) fermentation by equine microflora. Methods and Results: When fecal washed cell suspensions were incubated with any substrate amylolytics increased over time. However, at 24 h there were 10 and 1,000-fold more amylolytics with corn than wheat or oats, respectively. Predominant amylolytics isolated were Enterococcus faecalis (corn, wheat) and Streptococcus bovis (oats). GPC increased with any substrate, but decreased during stationary phase in oats only. Lactobacilli decreased during stationary phase with corn only. By 24 h, oats had more lactate-utilizers and lactobacilli and fewer GPC than corn and wheat. More gas was produced from oats or wheat than from corn. Conclusions: These results indicate that the growth of bacteria and fermentative capacity associated with starch metabolism is starch source dependent. Significance and Impact of the Study: This study demonstrates a relationship between starch source and gastrointestinal microbial changes independent of host digestion. However, future research is needed to evaluate the effect of starch source on the hindgut microbial community in vivo.