Dynamics of biofilm-forming Bacillus subtilis in Caenorhabditis elegans gut

Biofilm-forming Bacillus subtilis strain NCIB3610 has been reported to show biofilm-mediated beneficial effects when fed to Caenorhabditis elegans, including enhanced stress resistance, extended lifespan, and protection against neurotoxic agents. However, biofilm-forming B. subtilis presents multiple cell types, and thus a thorough characterization of B. subtilis cell dynamics in the C. elegans gut in vivo is necessary to understand mechanisms of the biofilm-mediated beneficial effects. In this study, we utilized an isogenic NCIB3610 strain expressing a biofilm reporter PtapA-mKate2 to monitor matrix-producing components in C. elegans gut. Interestingly, our results revealed that in live C. elegans gut, no B. subtilis biofilms were formed but only free spores and diffused fluorescence, which were likely from dietary matrix-producing cells crushed by the pharynx. Additional data showed that spores can geminate into motile cells but not matrix-producing cells in living C. elegans gut. Biofilm formation was found in gut after C. elegans death, indicating that mechanistically, life-dependent functions of the worm inhibit the formation of matrix-producing cells in gut. These findings resolved a key piece of puzzle in understanding the fundamental mechanisms of C. elegans-B. subtilis interaction and highlighted the importance of characterizing cell dynamics in vivo in host-microbe interaction research.