Cell density and mobility protect swarming bacteria against antibiotics.

Swarming bacteria move in multicellular groups and exhibit adaptive resistance to multiple antibiotics. Analysis of this phenomenon has revealed the protective power of high cell densities to withstand exposure to otherwise lethal antibiotic concentrations. We find that high densities promote bacterial survival, even in a nonswarming state, but that the ability to move, as well as the speed of movement, confers an added advantage, making swarming an effective strategy for prevailing against antimicrobials. We find no evidence of induced resistance pathways or quorum-sensing mechanisms controlling this group resistance, which occurs at a cost to cells directly exposed to the antibiotic. This work has relevance to the adaptive antibiotic resistance of bacterial biofilms.