Fig. 8: Spatial order of bacillaene inhibitory functions and its implications for the interspecies bacterial competition.

a Schematic of B. subtilis-E. coli colony interaction illustrating gradient-dependent inhibitory effects of bacillaene. In its competition with E. coli in biofilms, B. subtilis secretes the PK/NRP hybrid metabolite bacillaene, which diffuses away creating a distance-dependent concentration gradient. As a function of this gradient, the first bacillaene effect on E. coli biofilm cells is to inhibit curli assembly rendering cells “naked”, which occurs when cells are exposed to sub-bacteriostatic concentrations of bacillaene (upper panel). As B. subtilis and E. coli biofilms become closer, those E. coli cells that already lose their curli protection become exposed to higher bacillaene concentrations, which starts causing a second effect on E. coli cells, that is, to inhibit their proliferation (bacteriostatic effect) (lower panel). Thus, by combining in a single metabolite two inhibitory functions active at distinct concentrations, B. subtilis appears to have optimized a “weapon” to antagonize bacteria such as E. coli; it first renders cells devoid of their ECM protection and, secondly, once they are unshielded, it inhibits their growth. b Simplified schematic of RpoS-driven control of curli subunit expression and extracellular subunit assembly into amyloid fibers in E. coli cells distant from B. subtilis, not reached by bacillaene (upper panel). At sub-MIC concentrations, bacillaene inhibits extracellular assembly of curli subunits into amyloids fibers. E. coli cells react to this and other B. subtilis stresses trying to reinforce curli production by enhancing CsgB/A expression via a RpoS-mediated competition sensing response (lower panel).