Fig. 1: B. subtilis strongly inhibits curli-mediated morphogenesis of E. coli biofilms.

a Simplified schematic depicting key factors within the regulatory pathway that leads to curli subunit expression and the subsequent extracellular assembly of these subunits into amyloid fibers in E. coli cells. b, c Macrocolonies of E. coli W3110 grown on CR-containing salt-free LB agar (b) alone or (c) in close interaction with a macrocolony of B. subtilis 3610. The lower image in (b) shows at higher magnification the ringed and CR-stained patterns of a W3110 macrocolony that are dependent on curli. The lower image in (c) shows the absence of morphological development and CR staining in the E. coli macrocolony at the zone of interaction with B. subtilis. Scale bar in (b) represents 5 mm and is applicable to the image in (c). d W3110 macrocolonies grown in close proximity to paper discs containing concentrated cell-free supernatant (CCFS) of B. subtilis 3610 or culture medium (control). CCFS inhibits curli-dependent morphology and CR-staining patterns. e W3110 macrocolony grown on TS-containing salt-free LB agar in interaction with B. subtilis 3610. The cyan and yellow-boxed areas in the E. coli macrocolony indicate areas of the interaction and distant zones, respectively, that were examined in the cross-sections presented in (f) and (g). Cyan and yellow arrows serve as guides for the orientation of each zone analyzed in the respective cross-sections (the arrows point from the macrocolony center towards the border). f, g Merged fluorescence and phase contrast images of representative cross-sections through the W3110 macrocolony showing (f) the almost complete absence of curli at the interaction zone and (g) the abundance of TS-stained curli at the distant zone. Images reveal how the presence or absence of curli influence the micro and macrostructures of the colonies. Images at the right side are enlarged views of color-coded areas boxed in the cross-sections that show the amount and spatial arrangement of TS-stained curli fibers in each area. The white box in the enlarged-view image in (g) frames a representative basket-like structure. The spectral plots depict quantified patterns of TS fluorescence in respective cross-sections. Scale bars in (f, g) represent 5 μm.