Verticalization of bacterial biofilms

TL;DR

This study uncovers the physical mechanisms behind the vertical growth of bacterial biofilms, highlighting how cell length and mechanical instabilities influence biofilm structure and expansion.

Contribution

It introduces a combined agent-based and continuum model to explain biofilm verticalization and predicts how cell length affects biofilm morphology and growth dynamics.

Findings

Verticalization driven by mechanical instabilities varies with cell length.

Longer cells lead to faster, flatter biofilm expansion.

Chemical modulation of cell length alters biofilm development.

Abstract

Biofilms are communities of bacteria adhered to surfaces. Recently, biofilms of rod-shaped bacteria were observed at single-cell resolution and shown to develop from a disordered, two-dimensional layer of founder cells into a three-dimensional structure with a vertically-aligned core. Here, we elucidate the physical mechanism underpinning this transition using a combination of agent-based and continuum modeling. We find that verticalization proceeds through a series of localized mechanical instabilities on the cellular scale. For short cells, these instabilities are primarily triggered by cell division, whereas long cells are more likely to be peeled off the surface by nearby vertical cells, creating an "inverse domino effect". The interplay between cell growth and cell verticalization gives rise to an exotic mechanical state in which the effective surface pressure becomes constant throughout the growing core of the biofilm surface layer. This dynamical isobaricity determines the expansion speed of a biofilm cluster and thereby governs how cells access the third dimension. In particular, theory predicts that a longer average cell length yields more rapidly expanding, flatter biofilms. We experimentally show that such changes in biofilm development occur by exploiting chemicals that modulate cell length.