Gut Microbial Promotion of Energy Gain on Processed Diets

Many edible plants produce potent antimicrobial compounds as a defense against predation from bacteria and fungi. When these plant foods are eaten raw, these compounds could theoretically exert effects on the trillions of microorganisms inhabiting the gastrointestinal tract (gut microbiota). To test this hypothesis, we surveyed the composition and activity of the gut microbial communities of adult C57BL/6J mice (Mus musculus) fed controlled diets that varied in their antimicrobial loads as a result of cooking or supplementation. Diets of sweet potato (Ipomoea batatas) fed raw versus cooked produced dramatic changes in the composition of the distal gut microbiota, as indicated by 16S rRNA gene sequencing of fecal samples. Raw diets led to lower proportions of bacteria from the Firmicutes versus Bacteroidetes phyla, a microbial phenotype consistent with the weight loss observed in raw-fed hosts. Flow cytometry employing physiological stains indicated that raw diets were associated with increased rates of cell damage and lower proportions of highly active cells, a finding consistent with our observation of profoundly lower mRNA yields from gut microbial communities sampled from raw-fed versus cooked-fed hosts. To assess whether these effects were specifically attributable to food-derived compounds, we recently repeated the study with standard chow diets and water administered with or without one of two key antimicrobial compounds present in sweet potato: caffeic acid and chlorogenic acid. Our results suggest that such food-derived compounds can indeed promote bacterial cell damage within the gut, a result confirmed by in vitro assays of the effects of these compounds on gut microbial isolates. Our emerging results encourage a view beyond therapeutics in considering the role of xenobiotic compounds on host-microbial interactions in human health.