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, which can resolve each cell of the hundreds that compose a biofilm, 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. By combining this new multi-modal microscopy technique with a suite of experiments that modulate the biofilm-invader interaction, we uncover the physical and biological principles behind biofilms' defensive strategies, giving insights into bacterial ecology and disease prevention.