Capillary flow and mechanical buckling in a growing annular bacterial colony*

A growing bacterial colony is a dense suspension of an increasing number of cells. Starting by inoculating Pseudomonas aeruginosa over an annular area on an agar plate, we observe the growth and spread of the bacterial population, and model the process by consideration of physical effects that account for numerous features observed, such as edge accumulation, periodic protrusions, formation of bacterial droplets, and their growth, coalesce and surfing over the pre-wet agar surface. We conclude that fluid dynamics and elasto-mechanics together govern the bacterial colony pattern evolution. This study offers a clear example that physical effects account for large scale patterns that develop in growing bacterial colonies.