Role of PFOR in metronidazole resistance in clinical isolates. Two metronidazole (MTZ)-resistant CDI-associated strains (491858 and CD26A54_R) were complemented with the WT copy of nifJ. This caused a reduction in metronidazole agar MICs. MIC reductions were significant (**, P < 0.005; *, P < 0.05 for 491858 and CD26A54_R, respectively), as determined by unpaired t tests in GraphPad Prism, version 8.0. EV, empty vector. Strains 491858 and CD26A54_R carried Ala1018Val and Gly423Glu mutations, respectively, in PFOR.