Bacterial Swarms Recruit Cargo Bacteria To Pave the Way in Toxic Environments

Swarming bacteria are challenged by the need to invade hostile environments. Swarms of theflagellated bacterium Paenibacillus vortex can collectively transport other microorganisms. Here we show that P. vortex can invade toxic environ- ments by carrying antibiotic-degrading bacteria; this transport is mediated by a specialized, phenotypic subpopulation utilizing a process not dependent on cargo motility. Swarms of beta-lactam antibiotic (BLA)-sensitive P. vortex used beta-lactamase- producing, resistant, cargo bacteria to detoxify BLAs in their path. In the presence of BLAs, both transporter and cargo bacteria gained from this temporary cooperation; there was a positive correlation between BLA resistance and dispersal. P. vortex trans- ported only the most beneficial antibiotic-resistant cargo (including environmental and clinical isolates) in a sustained way. P. vortex displayed a bet-hedging strategy that promoted the colonization of nontoxic niches by P. vortex alone; when detoxify- ing cargo bacteria were not needed, they were lost. This work has relevance for the dispersal of antibiotic-resistant microorgan- isms and for strategies for asymmetric cooperation with agricultural and medical implications. IMPORTANCE Antibiotic resistance is a major health threat. We show a novel mechanism for the local spread of antibiotic resis- tance. This involves interactions between different bacteria: one species provides an enzyme that detoxifies the antibiotic (a ses- sile cargo bacterium carrying a resistance gene), while the other (Paenibacillus vortex) moves itself and transports the cargo. P. vortex used a bet-hedging strategy, colonizing new environments alone when the cargo added no benefit, but cooperating when the cargo was needed. This work is of interest in an evolutionary context and sheds light on fundamental questions, such as how environmental antibiotic resistance may lead to clinical resistance and also microbial social organization, as well as the costs, benefits, and risks of dispersal in the environment.