How do bacteria adapt their appendages to locomotion on a surface? To date, two main types of bacterial surface motility modes have been identified for P. aeruginsoa. Both flagella-mediated swarming motility and pili-mediated twitching motility have been extensively investigated. Using a soft matter physics-based technique to track social behavior of bacteria, we discovered a third type of motility, which occurs during early stages of biofilm formation: On a surface, bacteria orient themselves perpendicular to the surface and ‘walk’ upright using their type IV pili appendages with trajectories that efficiently explore all directions of a 2D surface. We find that bacterial ‘walking’ perpendicular to the surface and ‘crawling’ parallel to the surface represent two distinct foraging strategies on surfaces: the jagged ‘walking’ trajectories are optimized for rapid exploration of local micro-environments, with efficient area coverage, and the smoother ‘crawling’ trajectories are optimized for rapid motion in a fixed direction with efficient distance coverage.
Our new tracking method can delineate the full histories for every individual surface-associated cell in a movie and identify the type of motility appendage used at different times. The information content that we extract from a movie is ~105 times greater than that from traditional microscopy analysis methods, which look at a few cells at a time over a limited time period.