Buckling of growing bacterial chains

Bacteria usually grow in populations to survive in various environments. A yet unsolved issue is how mechanical forces at the cellular level regulate the morphogenesis of bacterial population. In this paper, we combine experiments and theoretical analysis to investigate the growth of bacterial chains constituted by rod-shaped Bacillus subtilis. Our experiments show that due to cellular proliferation-induced compressive forces, one-dimensional bacterial chains can buckle into a morphology similar to toppling dominoes. A rod–spring model which considers both cell–cell connection and cell–substrate interaction is established to study the buckling behaviors of bacterial chains. The critical features at the growth-induced instability of bacterial chains are determined by the energetic competition associated with elastic strain energy and cell–substrate bonding energy. An active rod method is further presented to simulate the bacterial growth dynamics from a single bacterium to a chain structure and, subsequently, to a two-dimensional biofilm, where Johnson–Kendall–Roberts theory is adopted to account for intercellular contact forces. This work suggests a novel mechanism of instability in the self-organization and expansion of bacterial biofilms.