Multiple causes, apart from genetic inheritance, predispose to the production and aggregation of amyloid-␤ (A␤) peptide and Alzheimer's disease (AD) development in the older population. There is currently no therapy or medicine to prevent or delay AD progression. One novel strategy against AD might involve the use of psychobiotics, probiotic gut bacteria with specific mental health benefits. Here, we report the neuronal and behavioral protective effects of the probiotic bacterium Bacillus subtilis in a Caenorhabditis elegans AD model. Aging and neuronal deterioration constitute important risk factors for AD development, and we showed that B. subtilis significantly delayed both detrimental processes in the wild-type C. elegans strain N2 compared with N2 worms colonized by the non-probiotic Escherichia coli OP50 strain. Importantly, B. subtilis alleviated the AD-related paralysis phenotype of the transgenic C. elegans strains CL2120 and GMC101 that express, in body wall muscle cells, the toxic peptides A␤ 3-42 and A␤ 1-42 , respectively. B. subtilis-colonized CL2355 worms were protected from the behavioral deficits (e.g., poor chemotactic response and decreased body bends) produced by pan-neuronal A␤ 1-42 expression. Notably, B. subtilis restored the lifespan level of C. elegans strains that express A␤ to values similar to the life expectancy of the wild-type strain N2 fed on E. coli OP50 cells. The B. subtilis proficiencies in quorum-sensing peptide (i.e., the Competence Sporulation Factor, CSF) synthesis and gut-associated biofilm formation (related to the antiaging effect of the probiotic) play a crucial role in the anti-AD effects of B. subtilis. These novel results are discussed in the context of how B. subtilis might exert its beneficial effects from the gut to the brain of people with or at risk of developing AD.

Bacterial biofilms are important in natural settings, biotechnology, and medicine. However, regulation of biofilm development and its persistence in different niches is complex and only partially understood. One key step during the biofilm life cycle is dispersal, when motile cells abandon the mature biofilm to spread out and colonize new niches. Here, we show that in the model bacteriumBacillus subtilisthe general stress transcription factor SigB is essential for halting detrimental overgrowth of mature biofilm and for triggering dispersal when nutrients become limited. Specifically, SigB-deficient biofilms were larger than wild-type biofilms but exhibited accelerated cell death, significantly greater sensitivity to different stresses, and reduced dispersal. Interestingly, the signal detected by SigB to limit biofilm growth was transduced through the RsbP-dependent metabolic arm of the SigB regulatory cascade, which in turn positively controlled expression of SinR, the master regulator of biofilm formation and cell motility. This novel SigB-SinR regulatory circuit might be important in controlling the fitness of biofilms (either beneficial or harmful) in diverse environments.IMPORTANCEBiofilms are crucial for bacterial survival, adaptation, and dissemination in natural, industrial, and medical systems. Sessile cells embedded in the self-produced extracellular matrix of the biofilm benefit from a division of labor and are protected from environmental insults. However, as the biofilm ages, cells become stressed because of overcrowding, starvation, and accumulation of waste products. How does the sessile biofilm community sense and respond to stressful conditions? Here, we show that inBacillus subtilis, the transcription factors SigB and SinR control whether cells remain in or leave a biofilm when metabolic conditions become unfavorable. This novel SigB-SinR regulatory circuit might be important for controlling the fitness of biofilms (either beneficial or harmful) in diverse environments.

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