It is believed that beneficial gut bacteria (probiotics) might positively contribute to the modulation of aging and the nervous system in their hosts. However, the mechanisms causing the effects due to probiotics remain poorly understood. In this way, the role of in vivo probiotic biofilm development during gut colonization and its effects on animal health and lifespan are poorly documented. For these reasons, we have investigated the ability of B. subtilis, a probiotic bacteria, to form a biofilm, to produce nitric oxide (NO) and to synthesize quorum sensing molecules in the C. elegans gut and we evaluated its effects on gut colonization, lifespan and in the modulation of the nervous system of Caenorhabditis elegans. The traslucent nematode C. elegans has proven to be an excellent model for the study of many central biological processes, including how the gut microbiota affects host physiology.The two wild-type B. subtilis strains used in this study were the domesticated and laboratory-reference strain JH642 and the isogenic undomesticated and wild B. subtilis strain NCIB3610. We also used mutant strains of this bacterium. N2 and mutant strains of C. elegans containing, as the food source, OP50 E. coli cells or B. subtilis as spores or vegetative cells or P. aeruginosa (PA14) were used in many essays. We developed lifespan experiments, thermotolerance and other stress essays. We cultured bacteria from worms, we studied biofilm development in the C. elegans gut, we used pulse-chase experiments to evaluate persistance and we measured NO and CFS under planktonic and biofilm conditions. Finally, we studied the intracellular routes that are involved in this bacteria-host interaction and the beneficial effects that this relationship produced in the nervous system of the host.Our work showed that B. subtilis spores are able to germinate, growth, make a biofilm, re-sporulate and persist in the intestine of C. elegans. Moreover, the proficiency of biofilm formation and the production of the anti-aging molecule NO and the quorum sensing pentapeptide CSF act separately to extend worm longevity. Furthermore, B. subtilis grown under biofilm-supporting conditions synthesizes higher levels of NO and CSF than under planktonic growth. We also could discover the routes involved in these effects and the property that this probiotic has to delay neurodegeneration produces by for example P. aeruginosa (PA14).In sum, B. subtilis can improve the health and lifespan of its host and its biofilm proficiency is a crucial factor.