Introduction: We identified a Na⁺-coupled transporter (SLC5A8) and a G-protein-coupled receptor (GPR109A) for butyrate, a short-chain fatty acid generated in the colon by bacterial fermentation of dietary fiber. GPR109A is coupled to Gi and decreases cellular levels of cAMP. cAMP is diarrheagenic as it inhibits the Na⁺/H⁺ and Cl^-/HCO3^-exchanger activities and stimulates the Cl^--channel activity. Butyrate may function as an antidiarrheal agent in the colon through its role as a substrate for SLC5A8 to promote Na⁺ absorption and as a ligand for GPR109A to reduce cellular levels of cAMP. Since butyrate is produced in the colonic lumen by bacterial fermentation, the presence or absence of bacteria in the colon might influence the expression of SLC5A8 and GPR109A.Hypothesis: SLC5A8 and GPR109A expression in the intestine/colon varies between conventional (CONV) and germ-free (GF) mice.Methods: Age-matched CONV and GF mice were obtained from Taconic and used for experiments upon arrival at our institution. GF mice were also kept in conventional conditions for 0.5-4 weeks for studies involving bacterial re-colonization. RNA and tissue sections were prepared from the intestinal tract and the expression of SLC5A8 and GPR109A was assessed by RT-PCR and immunohistochemistry. Differences in global gene expression among the CONV, GF, and re-colonized mice were explored by microarray analysis. Results: SLC5A8 and GPR109A mRNA and protein levels in colon and ileum were markedly reduced in GF compared to CONV mice, but the expression levels were completely reversed in the re-colonized GF mice. As a positive control, the expressions of SGLT1 and FIAF were simultaneously monitored. It’s known that SGLT1 mRNA levels are reduced while FIAF mRNA levels are increased in the ileum in GF compared to CONV mice. Our studies confirmed these earlier findings in the ileum and also showed a similar trend in the colon. DNA microarray analysis indicated altered expression of ~700 genes by more than 2-fold in GF compared to CONV mouse colon. These changes were reversed following re-colonization. Regarding electrolyte and water absorption, we found down-regulation of DRA, which codes for a Cl^-/HCO3^- exchanger, and aquaporin 4, which is the primary colonic water channel.Conclusions: These studies show that commensal bacteria promote the expression of several genes whose protein products are responsible for water and electrolyte absorption in the gut (SLC5A8, GPR109A, SGLT1, DRA, AQP4). Thus colonic bacteria are critical for regulating gut water and electrolyte absorption which could have a clinical impact relating to gut inflammation, infectious diarrhea and antibiotic therapy.