A coevolution experiment between and reveals parallel mutations that reduce antibiotic susceptibility.

One interference mechanism of bacterial competition is the production of antibiotics. Bacteria exposed to antibiotics can resist antibiotic inhibition through intrinsic or acquired mechanisms. Here, we performed a coevolution experiment to understand the long-term consequences of antibiotic production and antibiotic susceptibility for two environmental bacterial strains. We grew five independent lines of the antibiotic-producing environmental strain, E264, and the antibiotic-inhibited environmental strain, UW101, together and separately on agar plates for 7.5 months (1.5 month incubations), transferring each line five times to new agar plates. We observed that the ancestor could tolerate the -produced antibiotic through efflux mechanisms, but that the coevolved lines had reduced susceptibility. We then sequenced genomes from the coevolved and monoculture lines, and uncovered mutational ramifications for the long-term antibiotic exposure. The coevolved genomes from revealed four potential mutational signatures of reduced antibiotic susceptibility that were not observed in the evolved monoculture lines. Two mutations were found in : one corresponding to a 33 bp deletion and the other corresponding to a nonsynonymous mutation. A third mutation was observed as a 1 bp insertion coding for a RagB/SusD nutrient uptake protein. The last mutation was a G83R nonsynonymous mutation in acetyl-coA carboxylayse carboxyltransferase subunit alpha (AccA). Deleting the 33 bp from in the ancestor reduced antibiotic susceptibility, but not to the degree observed in coevolved lines. Furthermore, the mutation matched a previously described mutation conferring resistance to -produced thailandamide. Analysis of transposon mutants for thailandamide production revealed that thailandamide was bioactive against but also suggested that additional -produced antibiotics were involved in the inhibition of . This study reveals how multi-generational interspecies interactions, mediated through chemical exchange, can result in novel interaction-specific mutations, some of which may contribute to reductions in antibiotic susceptibility.