Antibiotic-induced recombination in bacteria requires the formation of double-strand breaks

Recombination is an essential process in bacterial drug resistance evolution. Fluoroquinolones, an important class of antibiotics, are known to stimulate recombination in Escherichia coli . On the other hand, when bacteria are exposed to sublethal concentrations of different antibiotic families, including strong inducers of the SOS system, which upregulates recombination pathways, there is no detectable change in the recombination rate. Here we explore why fluoroquinolones, but not any other tested antimicrobials, increase recombination rates. One feature that makes the fluoroquinolones different from other antibiotics is the generation of DNA double-strand breaks (DSBs). We tested whether other antimicrobials known for their ability to cause double-strand breaks, such as mitomycin C and bleomycin, are also affecting bacterial recombination rate. We found that both drugs indeed affected recombination rates in E. coli . Furthermore, there was a positive correlation between the number of DNA double-strand breaks and the recombination frequency. The manipulation of the level of DSBs directly impacted the recombination frequency. These results imply that recombination, at least in Gram-negative bacteria, is conservative, and even if multiple copies of a DNA sequence are available, the basal recombination rate is low. Our results highlight that homologous recombination is intended more for DNA damage repair than generating diversity, or at least that they occur at a very different scale. We also show that only antibiotics that cause DNA double-strand breaks can increase genetic diversity via recombination. The stimulation of recombination by DSBs-causing antimicrobials is an additional factor leading to the risk of antibiotic resistance evolution. ### Competing Interest Statement The authors have declared no competing interest.