Surface tension regulates the morphological evolution of a growing bacterial colony at an air-solid interface

There are several medically and industrially important contexts in which bacteria grow on surfaces to form biofilms at an interface between air and porous media. However, the dynamics associated with bacterial growth from a single cell and its evolution into a 3D biofilm at such interfaces remain largely unknown. Here, we study the early morphological evolution of an E. Coli colony growing on the surface of an agarose gel (open to air) as it grows from a single cell into a colony of over ~10,000 cells. We use laser scanning microscopy to measure the 3D shape of a growing biofilm and vary agarose gel concentration to systematically control for water availability around the cells. Starting from a single cell, we observe an initial growth phase where E. coli cells on the agarose surface primarily grow and divide in-plane in two-dimensions. Thereafter, we observe a distinctive out-of-plane morphological evolution, which we hypothesize is initiated by a mechanical instability arising from an interplay among growth-associated mechanical stresses, cell-cell and cell-substrate adhesion, and surface tension. Our observations suggest a model in which surface tension forces are important in regulating the overall shape and evolution of bacterial colonies growing at air-solid interfaces.