United we stand, divided we fall: Response to antibiotics in a bacterial swarm

Bacterial swarms consist of millions of bacterial cells collectively colonizing a nutrient rich agar plate. Pseudomonas aeruginosa is a bacterial species in which swarming is modulated by the interplay of surfactant driven fluid flow and chemotactic motility of individual cells, leading to the formation of a sparse branched pattern over the agar plate. The branches of the swarm do not intersect due to repulsive forces from the surfactant gradients. However, when an antibiotic drop is added near one of the growing branches, its growth gets arrested and the neighbouring branches intersect to join this branch. This is quite non-intuitive as the bacterial cells move towards the branch exposed to antibiotic as opposed to escaping the area to protect themselves. Moreover, this behaviour requires a complex signalling mechanism as the bacterial cells cannot directly sense the antibiotic as they lack the receptors specific to the antibiotic (gentamicin). We present experimental data and a mathematical model to unravel the mechanism behind this behaviour. Our findings reveal that stress on the bacterial community drives them into collaborating with each other to demonstrate collective resistance and avoidance of antibiotic toxicity.