Engineering Probiotics for Serotonin Production as a Potential Adjunctive Therapy for Depression

Abstract

Depression remains the main cause affecting global mental health, but current drug treatments are often limited by slow onset and significant side effects. As a potential alternative, we explored the possibility of programming bacterial hosts to directly synthesize serotonin (5-HT) in the gut. To this end, we constructed a synthetic operon containing tryptophan hydroxylase (TPH) and tryptophan decarboxylase (TDC) genes, which were fused by overlapping PCR and inserted under the control of the trc promoter in the pTrc99k plasmid. After transforming the plasmid into E. coli BL21 (DE3), we drove gene expression by gradient regulating IPTG concentration within the range of 0 to 10 mM. The SDS-PAGE electrophoresis results showed that both protein bands significantly increased with the increase of IPTG concentration and reached a plateau at approximately 5 mM. These results confirm that the dual enzyme 5-HT production module can operate normally in laboratory strains. However, applying this design to the field of probiotics is still a necessary next step, and we must further verify its actual production in the intestine.