Figure 1

Old macrocolony biofilms are producers of E. coli’s corona. (A) Typical volcano-like appearance (morphotype) of a 14-old-day macrocolony biofilm of E. coli K-12 MG1655 strain developed over a semisolid 0.6% ABE agar surface visualized by reflected light. (B) Schematic representation of characteristic structures observed in this macrocolony. EC, external colony corresponds to the superficial aerial, visible part of macrocolony developed on the semisolid agar surface. IB, represent the internal biofilm, an agar-entrapped formation, composed of "root" (R) and "corona" (C) developed inside semisolid agar. The root is the structure generated by bacterial "fingers" that penetrate inside the agar along the toothpick-punctured zone, while the corona (C) is the biofilm structure that develops in contact with the plastic surface (PS) of the base of the Petri dish. (C) Typical 14-old-day corona of E. coli K-12 MG1655 strain. (D-E) Close-up of the corona and a coronal spike. (F-J ) Zooming view of a coronal spike removed from a corona and placed under a microscope at different magnifications. (F) Part of a corona viewed to × 40, the box indicates the coronal spike removed and observed at different magnifications: (G) × 100 (H) × 400 (I-J) ×1000. ( K ) Typical 14-old-day E. coli K-12 BW25113 strain volcano-like colony and their corona ( L-M), (L) × 40 (M) × 100 ( N-O ). Typical 14-old-day corona of E. coli K-12 W3110 strain. (N) × 40 (O) × 100 magnifications. Each box (right) represents the enlargement region in the following image (left). Scale bars: (A, K) 0.5 cm (C) 0.1 cm (D) 500 μm (E, M, O) 100 μm (F) 350 μm (G) 150 μm (L, N) 200 μm (H) 40 μm (I-J) 20 μm.