Buckling instabilities in chaining bacterial colonies
Bacteria in the natural environment frequently grow as structured communities known as bacterial biofilms. The morphology of the colony is an emergent property, driven in part by the growth and activity of the constituents. Here, we are interested in the effect of pole-pole adhesion between constituents on the resulting colony dynamics and properties, in an effort to understand more about colonies of chaining bacteria such as \textit{Bacillus subtilis}. In this work, we investigate a 2D discrete element simulation of a growing bacterial colony composed of non-motile rod-shaped bacteria where daughter cells are able to `chain’ together with springs. We find that despite the simplicity of the model, the emergent dynamics and morphology of the colony are drastically altered. At small chain lengths, the classic mosaic of micro-domains is recovered where the colony is isotropic on large length-scales but locally is heterogeneous and composed of domains of aligned cells. As we increase the chain length, there is a crossover to a regime where the colony is able to collectively buckle, characterised by an oscillatory-type morphology and a peak in observable properties e.g. colony aspect ratio, density, micro-domain area etc.. Continuing to increase the chain length gives rise to the possibility of individual chains buckling due to active stresses within the chain overcoming the restoring elastic force of the links, leading to a winding, ribbon-like appearance of the colony and a collapse in the observable properties.
