Verticalization transition in <i>Vibrio cholerae</i> biofilms

Biofilms are groups of bacteria that adhere to and grow on surfaces. Recent advances in imaging technology allowed entire Vibrio cholerae biofilms to be observed at single-cell resolution in real time, revealing a growth program consisting of several architectural transitions. These early observations showed that cells are not arranged randomly within a colony, but instead grow from a branched, two-dimensional layer of founder cells into a three-dimensional structure with a vertically-aligned core. Here, we elucidate the physical mechanism of this transition using a combination of agent-based and continuum modeling. We find that the competing effects of cell growth and cell verticalization give rise to an exotic mechanical state in which the pressure becomes constant throughout the entire growing core of the colony. This “dynamical isobaricity” sets the velocity of surface expansion and thereby regulates how cells access the third dimension. In particular, our theory predicts that a longer average cell length yields more rapidly expanding, flatter colonies. We experimentally observed such changes in colony development by using chemicals that perturb cell length.