In a proof-of-concept experiment described in their latest paper, the team created a colony of Escherichia coli bacteria in which retrons responded to the presence of a chemical, flipping a switch in the E. coli genome that made it resistant to an antibiotic. This transformation did not happen to the same extent inside every E. coli cell in the colony, however. The higher the concentration of the triggering chemical, the greater was the proportion of cells that ended up antibiotic resistant.