“…At the outset of this work, there were ten known DfrB family members (DfrB1–11; there is no DfrB8, [ 6 , 7 , 8 , 9 , 10 , 11 , 12 ]). All procure high TMP resistance in E. coli (MIC > 600 µg/mL; K i ~0.38 to 1.3 mM), and most were originally identified in clinical samples [ 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. The turnover rates of DfrB enzymes for dihydrofolate reduction ( k cat = 0.20–0.41 s −1 ) are at least 100-fold lower than bacterial FolA (e.g., k cat = 230 s −1 for E. coli FolA); nonetheless, a low level of DfrB expression suffices to confer TMP resistance [ 13 , 14 ].…”