High nutrient availability reduces the diversity and stability of the equine caecal microbiota

Summary

For mammals, the function and stability of the gastrointestinal (GI) microbiota are important for health and survival. The GI microbiota supplies the host with volatile fatty acids (VFAs) (1, 2), modulates the host GI immune responses, and forms a barrier against invasion of pathogens (3). It is well established that diet modulates the GI microbiota (2), while our knowledge is limited with respect to how diet affects the functionality, stability, and diversity of the gut microbiota. A particularly intriguing unresolved question is how these factors are affected by postprandial nutrient availability. The GI microbiota is crucial for efficient nutrient utilisation in equines, since up to 60% of the forage is digested by microbial fermentation – mostly occurring in the hindgut (4, 5). The equine caecum is an enlargement of the hindgut, containing a highly complex microbial ecosystem mainly dominated by members of the bacterial phyla Firmicutes and Bacteroides (6, 7). The effects of a single meal on the hindgut microbiota and fermentation pattern will be most significant in the caecum (8). Horse forage is commonly supplemented with whole oats to increase the energy intake. Whole oats consists of a core of easily fermentable starch that can partly be defined as resistant starch (9–11), since the starch-rich endosperm is encapsulated in a hull delaying the access of digestive enzymes to the starch (12). Thus, if not entirely chewed, a part of the starch given in the oat diet will escape small intestinal digestion, being a source for rapid hindgut fermentation. The aim of our work was to evaluate the effect of high and low available nutrient diets on the equine caecal microbiota function and stability. We did this in an in vivo experiment with horses fed hay (low and slower nutrient availability) or a hay/whole oats diet (high and more rapid nutrient availability). We used 16S rRNA gene deep sequencing and direct mixed sequencing for determining the microbiota composition in combination with measurements of VFA, NH3N, and pH for functional analyses of the caecum. We present results showing that the high available nutrient diet reduces both the diversity and the temporal stability of the caecal microbiota. These results are discussed in the light of general ecological theories and potential health consequences.