Although recent studies have confirmed that endogenous 5-HT has a minor role in GI-motility and transit in vivo, our data here demonstrate that a genetically engineered probiotic strain (EcN-5-HT) producing 5-HT is able to significantly improve intestinal motility in a murine constipation model (Figure 6). The gene discussed is HTR5A; the disease is constipation disorder.