The great divide: rhamnolipids mediate separation between P. aeruginosa and S. aureus

The coexistence of multiple bacterial species during infection can have significant impacts on pathogenesis. Pseudomonas aeruginosa and Staphylococcus aureus are opportunistic bacterial pathogens that can co-infect hosts and cause serious illness. The factors that dictate whether one species will outcompete the other or whether the two species can coexist are not fully understood. We investigated the role of surfactants in the interactions between these two species on a surface that enables P. aeruginosa to swarm. We found that P. aeruginosa swarms are repelled by colonies of clinical S. aureus isolates, creating physical separation between the two strains. This effect was abolished in mutants of S. aureus that were defective in the production of phenol-soluble modulins (PSMs), which form amyloid fibrils around wild-type colonies. We investigated the mechanism that establishes physical separation between the two species using the Imaging of Reflected Illuminated Structures (IRIS) method, which tracks the flow of the rhamnolipid surfactant layer produced by P. aeruginosa. We found that PSMs produced by S. aureus deflected the rhamnolipid surfactant layer flow, which in turn, altered the direction of P. aeruginosa swarms. These findings show that rhamnolipids mediate physical separation between P. aeruginosa and S. aureus, which enables these species to coexist in distinct microenvironments. Additionally, we found that a Bacillus subtilis surfactant and abiotic hydrophobic molecules repelled P. aeruginosa swarms through surfactant deflection. Our results suggest that surfactant interactions could have major impacts on bacteria-bacteria and bacteria-host relationships. In addition, our findings uncover a mechanism responsible for P. aeruginosa swarm development that does not rely on sensing but instead is guided largely by the flow of the surfactant layer and its boundaries.