Date of Award


Document Type


Degree Name

Master of Science (MS)


Microbiology and Molecular Genetics

First Advisor

Gary Mawe


It has become increasingly clear that there is bidirectional communication between the microbes that exist in the lumen of the gastrointestinal (GI) tract and the nervous system. Research within the last 20 years has revealed GI bacterial metabolites to have a significant effect on gut motility, and some of these actions involve influencing serotonin signaling in the epithelial layer of the intestines. Serotonin, abbreviated as 5- HT, has many functions within the gut including propulsive and segmentation motility, vasodilation, and epithelial cell secretion. There are several bacterial species that have been discovered to synthesize one of 5-HT’s precursory molecules, including Bacillus subtilis, which produces tryptophan. Prior data obtained from the Mawe lab has suggested that administering a short daily course of B. subtilis strain R0179 spores to mice promotes GI motility.The effects of another metabolite, tryptamine, on 5-HT signaling have also been investigated. Tryptamine is a derivative of tryptophan and has been found to stimulate the release of 5-HT in the mammalian GI tract. Moreover, it has recently been discovered that tryptamine may also bind to and activate 5-HT receptors in the gut, primarily 5- HT4R. 5-HT4Rs are highly expressed on epithelial cells in the colon, and activation of these receptors has been associated with increased prokinetic effects in the GI tract. Bacterial species have been identified that can produce tryptamine in the gut via tryptophan decarboxylase, which converts tryptophan to tryptamine. Kashyap and colleagues have recently determined that administering a modified Bacteroides thetaiotomicron strain capable of synthesizing tryptophan decarboxylase (Trp D+) to germ-free mice accelerated GI transit times by activating 5-HT4Rs and therefore promoting colonic secretion. However, the prokinetic effects of B. thetaiotomicron Trp D+ on conventional mice with normal gut microbiota are currently unknown as well as its effects in probiotic formulations. The primary aim of this study was to further understand how administering B. thetaiotomicron Trp D+ in combination to B. subtilis in mice may affect GI motility by modulating serotonin signaling. In this study, C57/BL6 mice approximately 8 weeks of age were utilized for several motility assays measuring whole GI transit time, colonic transit time, and fecal water content. Mice were either administered two treatments of B. thetaiotomicron Trp D+ one week apart (day 0 and day 7) followed by a one-week daily course of B. subtilis R0179 (day 7-14), two treatments of B. thetaiotomicron WT one week apart (day 0 and day 7) followed by a one-week daily course of B. subtilis R0179 (day 7-14), B. thetaiotomicron Trp D+ alone (day 0 and day 7), B. subtilis R0179 alone (day 7-14) or a vehicle control. While the mice receiving B. thetaiotomicron Trp D+ alone and B. thetaiotomicron WT with B. subtilis R0179 did not have any significant changes in while GI or colonic transit times, they did have a lower fecal water content indicating higher levels of water absorption in the gut. The mice receiving both B. thetaiotomicron Trp D+ and B. subtilis R0179 similarly demonstrated lower fecal water content. Interestingly, mice receiving both bacterial strains had significantly slower GI transit times yet faster colonic transit times. However, notably the mice receiving both bacteria did not have significantly faster colonic transit times than the mice receiving B. subtilis R0179 alone. In conclusion, this study revealed that B. thetaiotomicron Trp D+ administration may not be an effective method for enhancing GI motility. As probiotics are becoming more widely considered as a treatment for motility disorders including IBS, it is important to understand how various bacterial strains may impact the gut and serotonin signaling within. Overall, this study provides valuable insight in how to possibly improve probiotic formulations.



Number of Pages

81 p.