Necrosignaling: Cell death triggers antibiotic survival pathways in bacterial swarms

Swarming is form of collective bacterial motion enabled by flagella on the surface of semi-solid media1. Many bacterial species exhibit non-genetic adaptive resistance to a broad range of antibiotics only when swarming (SR)2-4. While the swarming population as a whole survives antibiotic challenge, it nonetheless sustains considerable cell death5. In this study focused mainly on E. coli, an initial analysis of antibiotic-induced killing patterns of swarm vs planktonic cells indicated that death of a sub-population is beneficial to the swarm in promoting SR. Introduction of pre-killed cells into a swarm indeed enhanced SR, allowing us to purify the SR factor from killed cells, and to establish that cell death is directly involved in SR. The SR-enhancing factor was identified to be AcrA, a periplasmic component of a tripartite RND efflux pump6, of which the outer membrane component TolC is shared by multiple drug efflux pumps7. We show that AcrA stimulates drug efflux in live cells by interacting with TolC from the outside. This stimulus acts as a signal to activate efflux in the short term, and to induce the expression of other classes of efflux pumps in the long term, amplifying the response and establishing SR. We call this phenomenon necrosignaling, and show the existence of species-specific necrosignaling in both Gram-positive and Gram-negative bacteria. Our results have implications for the adaptive resistance of other surface-associated bacterial collectives such as biofilms. Given that such resistance is known to be an incubator for evolving genetic resistance8, our findings may also be relevant to chemotherapy-resistant cancers.