Giardia intestinalis is the most important and common diarrhea-causing parasitic protozoa worldwide with growing clinical relevance in public health. There are many documented cases of G. intestinalis resistance to metronidazole (MZ). Pyruvate: ferredoxin oxidoreductase (PFOR), the membrane-localized enzyme, plays a key role in the development of resistance to drugs. The aim of the present study was to evaluate the difference in the levels of PFOR gene expression between MZ-resistant and

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... eptible strains of G. intestinatlis. Methodology: From 159 samples with G. intestinalis cysts, 48 strains were successfully cultivated. Using specific pair primers, PFOR gene expressions were estimated in different groups of Giardia. The polymerase chain reaction (PCR) data were analyzed with Bayesian analysis of qRT-PCR data using MCMC.qpcr package, with relative expression software tool (REST) and quantitative PCR CopyCount web source. Results: In the group of Giardia with minimum inhibitory concentration (MIC) of 6.3 µM, the level of PFOR gene expression was downregulated and compared with controls, differed by 1.5 to 2.8 times. At the same time, there was no significant difference in PFOR gene expression between the control (susceptible) group and the group with MIC of 3.2 µM. Conclusions: Though there is association between PFOR gene expression and metronidazole resistance of Giardia intestinalis, the level of PFOR gene expression cannot be a strong genetic marker to predict level of resistance to metronidazole based on MICs.