Bacteroides ovatus Influences the Levels of Intestinal Neurotransmitters in a Gnotobiotic Model

Abstract

Background Microbial metabolites and colonization have been associated with the levels and turn-over of a wide range of host neurotransmitters. Although animal models have been paramount in linking microbes to fundamental gastrointestinal and neural processes, the majority of current work has focused on complete microbial communities. The complex nature of these communities’ obscures which species drive neurotransmitter changes and makes it challenging to identify specific gut-brain-axis pathways. Previous studies have identified that Bacteroides, a dominant member of the human gut microbiome, can influence the gut-brain-axis; although the exact pathways of communication remain unclear. We hypothesized that Bacteroides species could influence gut neurotransmitter levels, initiating communication between distant organs. Methods & Result We grew B. ovatus, a representative human gut commensal, in a fully defined bacteria media, ZMBI, and examined short chain fatty acids and neurotransmitters by targeted liquid chromatography-tandem mass spectrometry (LC-MS/MS). We found that B. ovatus generates high levels of acetic acid and propionic acid, and relatively low concentrations of isobutyric acid and isovaleric acid. Pathway analysis revealed that B. ovatus consumed tryptophan and glutamate, and synthesized glutamine and GABA, potent neurotransmitters. We did not observe serotonin, 5-HIAA, melatonin, 5-hydroxytryptophan, dopamine, norepinephrine, or epinephrine in any of the samples. To determine how B. ovatus secreted products influenced neurotransmitter release from the intestinal epithelium, we microinjected intestinal organoids with B. ovatus supernatant. B. ovatus injected organoids were also found to have increased concentrations of tyrosine and tyramine compared to media control. Interestingly, B. ovatus was not found to stimulate host serotonin, melatonin, 5-HIAA or 5-hydroxytryptophan. Finally, we mono-associated germ-free adult C57BL/6J mice with the commensal B. ovatus for 17 days and examined colonic short chain fatty acids and neurotransmitters levels. Similar to B. ovatus supernatant, we observed acetic acid, propionic acid, isobutyric acid and isovaleric acid in gnotobiotic intestines. Complementing our organoid experiments, the concentrations of serotonin, melatonin, 5-HIAA and 5-hydroxytryptophan were not changed in B. ovatus mono-associated mice. However, B. ovatus mono-associated mice did increase intestinal tyrosine, glutamate, glutamine and GABA concentrations compared to germ-free controls. Conclusions These results highlight novel connections between microbial colonization and neurotransmitters. Our data indicates that B. ovatus selectively influences intestinal neurotransmitters levels, particular those of the GABA/Glu/Gln pathway, which likely has far reaching implications on human health.