Serotonin Exposure Improves Stress Resistance, Aggregation, and Biofilm Formation in the Probiotic Enterococcus faecium NCIMB10415

The role of the microbiota–gut–brain axis in maintaining a healthy status is well recognized. In this bidirectional flux, the influence of host hormones on gut bacteria is crucial. However, data on commensal/probiotics are scarce since most reports analyzed the effects of human bioactive compounds on opportunistic strains, highlighting the risk of increased pathogenicity under stimulation. The present investigation examined the modifications induced by 5HT, a tryptophan-derived molecule abundant in the intestine, on the probiotic Enterococcus faecium NCIMB10415. Specific phenotypic modifications concerning the probiotic potential and possible effects of treated bacteria on dendritic cells were explored together with the comparative soluble proteome evaluation. Increased resistance to bile salts and ampicillin in 5HT-stimulated conditions relate with overexpression of specific proteins (among which Zn-beta-lactamases, a Zn-transport protein and a protein involved in fatty acid incorporation into the membrane). Better auto-aggregating properties and biofilm-forming aptitude are consistent with enhanced QS peptide transport. Concerning interaction with the host, E. faecium NCIMB10415 enhanced dendritic cell maturation, but no significant differences were observed between 5HT-treated and untreated bacteria; meanwhile, after 5HT exposure, some moonlight proteins possibly involved in tissue adhesion were found in higher abundance. Finally, the finding in stimulated conditions of a higher abundance of VicR, a protein involved in two-component signal transduction system (VicK/R), suggests the existence of a possible surface receptor (VicK) for 5HT sensing in the strain studied. These overall data indicate that E. faecium NCIMB10415 modifies its physiology in response to 5HT by improving bacterial interactions and resistance to stressors.