The chemotherapeutic drug methotrexate selects for antibiotic resistance

Abstract Understanding drivers of antibiotic resistance evolution is fundamental for designing optimal treatment strategies and interventions to reduce the spread of antibiotic resistance. Various cytotoxic drugs used in cancer chemotherapy have antibacterial properties, but how bacterial populations are affected by these selective pressures is unknown. Here we test the hypothesis that the widely used cytotoxic drug methotrexate affects the evolution and selection of antibiotic resistance through the same mechanisms as the antibiotic trimethoprim. We show that methotrexate can select for trimethoprim resistance determinants located on the chromosome or a plasmid in clinical strains of Escherichia coli . Additionally, methotrexate can co-select for virtually any antibiotic resistance determinant when present together with trimethoprim resistance on a multidrug-resistance clinical plasmid. These selective effects occur at concentrations 40- to >320-fold below the methotrexate minimal inhibitory concentration for E. coli , suggesting a selective role of methotrexate chemotherapy for antibiotic resistance in patients that strongly depend on effective antibiotic treatment. Significance statement The presented data show that methotrexate has the potential to select for virtually any given antibiotic resistance gene when genetically linked to trimethoprim resistance. This study highlights the need for increased awareness of the presence of acquired antibiotic resistance determinants in the gut of patients with impaired immunity undergoing methotrexate treatment to preserve the effects of downstream antibiotic treatments.