Low-Level Antimicrobials in the Medicinal Leech Select for Resistant Pathogens That Spread to Patients

Fluoroquinolones (FQs) and ciprofloxacin (Cp) are important antimicrobials that pollute the environment in trace amounts. Although Cp has been recommended as prophylaxis for patients undergoing leech therapy to prevent infections by the leech gut symbiont , a puzzling rise in Cp-resistant (Cp ) infections has been reported. We report on the effects of subtherapeutic FQ concentrations on bacteria in an environmental reservoir, the medicinal leech, and describe the presence of multiple antibiotic resistance mutations and a gain-of-function resistance gene. We link the rise of Cp isolates to exposure of the leech microbiota to very low levels of Cp (0.01 to 0.04 µg/ml),