Metronidazole and Giardia : in vitro viability assay under microaerophilic conditions indicates a multifactorial basis for metronidazole treatment failure

We developed a simple 24-hour viability assay to evaluate the tolerance of the parasite Giardia intestinalis to metronidazole (MTZ) in vitro . Our findings emphasize the importance of test conditions and evaluation parameters. Using this assay with minimum lethal concentration (MLC) as an evaluation parameter, we identified two isolates that are naturally tolerant to MTZ. Natural MTZ tolerance was detected only under microaerophilic conditions, as both isolates were susceptible to MTZ under anaerobic conditions. These isolates were derived from stool samples of patients with MTZ-refractory giardiasis, suggesting that the parasite tolerance was responsible for the failure of MTZ treatment. However, two other isolates from patients with MTZ-refractory giardiasis were susceptible under both test conditions, indicating that MTZ-refractory giardiasis likely has a multifactorial cause. HIGHLIGHTS In vitro test conditions are significant to assess metronidazole tolerance in Giardia In vitro test should be done under both anaerobic and microaerophilic conditions MLC, but not IC 50 , is a valuable parameter for evaluating MTZ tolerance Failure to treat giardiasis with metronidazole probably has a multifactorial cause