B. subtilis GS67 Protects C. elegans from Gram-Positive Pathogens via Fengycin-Mediated Microbial Antagonism

Studies on Caenorhabditis elegans have provided detailed insight into host-pathogen interactions [1–7]. Usually, the E. coli strain OP50 is used as food source for laboratory studies, but recent work has shown that a variety of bacteria have dramatic effects on C. elegans physiology, including immune responses [8–18]. However, the mechanisms by which different bacteria impact worm resistance to pathogens are poorly understood. Although pathogen-specific immune priming is often discussed as a mechanism underlying such observations [13, 14], interspecies microbial antagonism might represent an alternative mode of action. Here, we use several natural Bacillus strains to study their effects on nematode survival upon pathogen challenge. We show that B. subtilis GS67 persists in the C. elegans intestine and increases worm resistance to Gram-positive pathogens, suggesting that direct inhibition of pathogens might be the primary protective mechanism. Indeed, chemical and genetic analyses identified the lipopeptide fengycin as the major inhibitory molecule produced by B. subtilis GS67. Specifically, a fengycin-defective mutant of B. subtilis GS67 lost inhibitory activity against pathogens and was unable to protect C. elegans from infections. Furthermore, we found that purified fengycin cures infected worms in a dose-dependent manner, indicating that it acts as an antibiotic. Our results reveal a molecular mechanism for commensal-mediated C. elegans protection and highlight the importance of interspecies microbial antagonism for the outcome of animal-pathogen interactions. Furthermore, our work strengthens C. elegans as an in vivo model to reveal protective mechanisms of commensal bacteria, including those relevant to mammalian hosts. •Natural Bacillus isolates protect C. elegans from B. thuringiensis DB27 infection•B. subtilis GS67 persists in C. elegans gut and inhibits DB27•Lipopeptide fengycin is the major inhibitory molecule of B. subtilis GS67•Fengycin mediates B. subtilis GS67 rescue of infected worms Iatsenko et al. show that B. subtilis protects C. elegans from infections via antibiotic-mediated pathogen inhibition. This highlights the importance of interspecies commensal-pathogen interplay for the outcome of host-pathogen interactions and strengthens C. elegans as an in vivo model to reveal protective mechanisms of commensal bacteria.