How do biofilms defend public goods? Mechanisms of protection against planktonic invasion

Many bacteria in nature exist in biofilms -- surface-associated colonies built around a structural polymer network produced by the embedded cells. This polymer network protects cells against physical and chemical intrusions, giving biofilm-dwelling cells a survival advantage at a significant metabolic cost. Mounting evidence shows that biofilms can also protect these public goods against invading free-swimming cells, even of the same species. However, the mechanism that biofilms use to protect themselves is unclear. Using the model biofilm forming species Vibrio cholerae -- the causative agent of the pandemic disease cholera -- we combine confocal microscopy that can resolve biofilms containing hundreds of cells at single-cell resolution with digital in-line holography, a technique that can reconstruct three-dimensional trajectories of cells swimming at speeds in excess of 100 microns per second. These methods capture interactions between biofilms and swimming cells that reveal multiple layers of defensive strategies, giving insights into bacterial ecology and disease prevention.