Pseudomonas aeruginosa PA14 Raw sequence reads. Pseudomonas aeruginosa PA14

In this study, we explore the antibiotic treatment conditions under which cost-free resistance arises, using experimental evolution of the bacterium Pseudomonas aeruginosa and the quinolone antibiotic ciprofloxacin. We find that intermittent antibiotic treatment leads to the evolution of cost-free resistance, and demonstrate that compensatory evolution is the mechanism responsible for cost-free resistance. Our results suggest that discontinuous administration of antibiotic may be contributing to the high levels of antibiotic resistance currently found worldwide.