Resistance of Trichomonas vaginalis to metronidazole: Report of the first three cases from Finland and optimization of in vitro susceptibility testing under various oxygen concentrations

Trichomonas vaginalis is a globally common sexually transmitted human paras ite. Many strains of T. vaginalis from around the world have been described to be resistant to the current drug of choice, metronidazole. However, onl y a few cases of metronidazole resistance have been reported from Europe. T he resistant strains cause prolonged infections which are difficult to trea t. T. vaginalis infection also increases the risk for human immunodeficienc y virus transmission. We present a practical method for determining the res istance of T. vaginalis to 5-nitroimidazoles. The suggested method was deve loped by determining the MICs and minimal lethal concentrations (MLCs) of m etronidazole and ornidazole for T. vaginalis under various aerobic and anae robic conditions. Using this assay we have found the first three metronidaz ole-resistant strains from Finland, although the origin of at least one of the strains seems to be Russia. Analysis of the patient-derived and preciou sly characterized isolates showed that metronidazole-resistant strains were also resistant to ornidazole, and MLCs for all strains tested correlated w ell with the MICs. The suggested MICs of metronidazole for differentiation of sensitive and resistant isolates are >75 mu g/ml in an aerobic 24-h assa y and >15 mu g/ml in an anaerobic 48-h assay.