Extended Data Fig. 1: Proteus mirabilis colonies at the early stage of biofilm development with prominent production of extracellular polymer matrix.

(a) Phase-contrast image of a circular disk-shaped P. mirabilis colony grown for 14 hr at 37 °C after inoculation with overnight culture (Methods). Scale bar, 500 μm. (b) Fluorescent image of extracellular amyloid fibrils matrix labelled by Thioflavin T (Methods) in the P. mirabilis biofilm shown in panel a. The outer rim of P. mirabilis colonies at this development stage is mostly occupied by immotile cells that have transitioned to the sessile state but expressed little extracellular matrix. The width of this outer immotile rim varies from tens to hundreds of µm across different colonies; in the case of panel a, the immotile rim ranges from radius R = ~860 µm (measured from the colony centre) to R = 1154 µm (i.e, the colony edge), spanning a width of ~300 µm. The inner region of the colonies with Thioflavin T fluorescence (that is, the biofilm region; enclosed by the dashed circle in panels a,b) is where we choose to study and refer to as early-stage biofilm or bacterial active solid, such as the fields in main text Figs. 1c,d and 3a. The immotile outer rim of the colony serves as the lateral spatial confinement for the bacterial active solid. Scale bar, 500 μm. (c) Enlarged view of the centre of panel b. Scale bar, 100 μm. (d) Phase-contrast images of oval-shaped P. mirabilis colonies with various values of the eccentricity. Scale bars, 500 μm. The field in main text Fig. 2a corresponds to the stripe region enclosed by the dashed rectangle in panel d.