The plant host induces antibiotic production to select the most beneficial colonizers

Microbial ecosystems tightly associated with a eukaryotic host are wide-spread in nature. The genetic and metabolic networks of the eukaryotic hosts and the associated microbes have co-evolved to form a symbiotic relationship. Both the Gram-positive Bacillus subtilis and the Gram-negative Serratia plymuthica can form biofilms on plant roots, and thus can serve as a model system for the study of interspecies interactions in a host-associated ecosystem. We found that B. subtilis biofilms expand collectively and asymmetrically towards Serratia plymuthica , while expressing a non-ribosomal antibiotic bacillaene and an extracellular protease. As a result, B. subtilis biofilms out-competed S. plymuthica for a successful colonization of the host. Strikingly, the plant host could enhance the efficiency of this killing by inducing bacillaene synthesis. In turn, B. subtilis biofilms increased the resistance of the plant host to pathogens. These results provide an example of how plant-bacterium symbiosis promotes the immune response of the plant host and the fitness of the associated bacteria. Importance Our study sheds a mechanistic light on how multicellular biofilm units compete and successfully colonize a eukaryote host, using B. subtilis microbial communities as our lens. The microbiota and its interactions with its host play various roles in the development and prevention of diseases. Using competing beneficial biofilms that are essential microbiota members on the plant host, we found that B. subtilis biofilms activate collective migration to capture their prey followed by non-ribosomal antibiotic synthesis. Plant hosts increase the efficiency of antibiotic production by B. subtilis biofilms as they activate the synthesis of polyketides, therefore our study provides a first evidence to a mechanism by which the host can indirectly select for beneficial microbiota members.