The inactive full-length form of GLP-1(1–37) stimulates conversion of both rat and human intestinal epithelial cells into insulin-secreting cells. We investigated whether oral administration of human commensal bacteria engineered to secrete GLP-1(1–37) could ameliorate hyperglycemia in a rat model of diabetes by reprogramming intestinal cells into glucose-responsive insulin-secreting cells. Diabetic rats were fed daily with human lactobacilli engineered to secrete GLP-1(1–37). Diabetic rats fed GLP-1-secreting bacteria showed significant increases in insulin levels and, additionally, were significantly more glucose tolerant than those fed the parent bacterial strain. These rats developed insulin-producing cells within the upper intestine in numbers sufficient to replace ∼25–33% of the insulin capacity of nondiabetic healthy rats. Intestinal tissues in rats with reprogrammed cells expressed MafA, PDX-1, and FoxA2. HNF-6 expression was observed only in crypt epithelia expressing insulin and not in epithelia located higher on the villous axis. Staining for other cell markers in rats treated with GLP-1(1–37)-secreting bacteria suggested that normal function was not inhibited by the close physical proximity of reprogrammed cells. These results provide evidence of the potential for a safe and effective nonabsorbed oral treatment for diabetes and support the concept of engineered commensal bacterial signaling to mediate enteric cell function in vivo.