Territorial Scent Marking by Rhizosphere Communities

The soil bacterium Bacillus subtilis forms beneficial biofilms on the surface of plant roots, preventing the growth of bacterial and fungal pathogens, and can serve as a biocontrol agent. Understanding the mechanisms deployed by B. subtilis to outcompete other rhizosphere-dwelling bacteria and successfully colonize the host is thus of tremendous agricultural importance. In this work, we establish an ecological role of airborne bioactive molecules produced by established B. subtilis communities during competition. Specifically, B. subtilis volatiles can dramatically inhibit the formation of biofilms formed by the same species and by a phylogenetically distinct pathogen sharing the same ecological niche enteropathogenic Escherichia coli (EPEC). A critical mass of cells was required to achieve long-distance inhibition. Furthermore, the volatiles served as signal molecules as they specifically decreased the expression of extracellular matrix genes for both species. Structurally related commercial volatiles mimicked the effect of physiologically produced volatiles that were identified by GC-MS. Interestingly, the ability of some volatiles to inhibit the growth of a competitor is growth-condition dependent. For instance, ammonia, a primary non-organic volatile produced by B. subtilis, could inhibit biofilm formation of B. subtilis and EPEC on defined media, but not on rich media. In conclusion, we describe an efficient mechanism for communication between physically separated rhizosphere communities allowing them to sense and response to their competitors over a large distance.