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Enterococcus faecium is an important nosocomial pathogen,particularly it developed resistance to multiple antibiotics including vancomycin.Nitrofurantoin is active against E.faecium and E.faecalis,and it retained its activity against vanA-and vanB-positive isolates.Although the specific mode of action of nitrofurantoin is still unknown,the strains resistant and susceptible to nitrofurantoin differ in their ability to reduce the compounds.We obtained a serial of Ri(f) mutant of E.faecium in the previous study (unpublished data).We observed that the mutant (B42-R7,rpoB H489D) exhibited enhanced sensitivity to nitrofurantoin when compared with its isogenic parent (B42).We determined the nitrofurantoin MIC,growth rate and ROS level to investigate the observation in more detail.Ri(f) mutant of E.faecium that was sensitive to nitrofurantoin,showed lower growth rate and higher ROS level.We also compared the proteomic profiles of E.faecium B42 and its Ri(f) mutant B42-R7.The induced nitroreductase in B42-R7 might explain that why B42-R7 became susceptible to nitrofurantoin.