Morphogenesis and fractal dimension of bacterial pellicles

Bacteria cells can self-organize into structured communities at phase boundaries, known as biofilms. At liquid-liquid and liquid-air interfaces, these soft, living materials of cells and extracellular polysaccharides – called pellicles – confer survival advantages and protection against environmental insults to the community. These benefits are not attainable by individual planktonic cells. The mechanics driving pellicle formation and morphology are not understood. Here, using a home-made adaptive stereoscope instrument and fluorescent microscopy, we identify a series of mechanical and architectural transitions in Vibrio cholerae pellicles at a liquid-oil interface. There are three distinct stages: emergence of founding colonies, onset of primary and secondary wrinkling instabilities, and a fractal-order increase in complexity. We show that although cells in a pellicle share a habitat, founder colonies remain monoclonal, hence the community maintains spatial and genetic heterogeneity.