Indole Derivatives Maintain the Status Quo Between Beneficial Biofilms and Their Plant Hosts.

Biofilms formed by bacteria on plant roots play an important role in maintaining an optimal rhizosphere environment that supports plant growth and fitness. Bacillus subtilis is a potent plant growth promoter, forming biofilms that play a key role in protecting the host from fungal and bacterial infections. In this work, we demonstrate that the development of B. subtilis biofilms is antagonized by specific indole derivatives that accumulate during symbiotic interactions with plant hosts. Indole derivatives are more potent signals when the plant polysaccharide xylan serves as a carbon source, a mechanism to sustain beneficial biofilms at a biomass that can be supported by the plant. Moreover, B. subtilis biofilms formed by mutants resistant to indole derivatives become deleterious to the plants due to their capacity to consume and recycle plant polysaccharides. These results demonstrate how a dynamic metabolite-based dialogue can promote homeostasis between plant hosts and their beneficial biofilm communities.

Toxin-Antitoxin systems eliminate defective cells and preserve symmetry in Bacillus subtilis biofilms.

Toxin-antitoxin modules are gene pairs encoding a toxin and its antitoxin, and are found on the chromosomes of many bacteria, including pathogens. Here, we characterize the specific contribution of the TxpA and YqcG toxins in elimination of defective cells from developing Bacillus subtilis biofilms. On nutrient limitation, defective cells accumulated in the biofilm breaking its symmetry. Deletion of the toxins resulted in accumulation of morphologically abnormal cells, and interfered with the proper development of the multicellular community. Dual physiological responses are of significance for TxpA and YqcG activation: nitrogen deprivation enhances the transcription of both TxpA and YqcG toxins, and simultaneously sensitizes the biofilm cells to their activity. Furthermore, we demonstrate that while both toxins when overexpressed affect the morphology of the developing biofilm, the toxin TxpA can act to lyse and dissolve pre-established B. subtilis biofilms.