Genetics of Nitrofurazone Resistance in Escherichia coli

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Abstract
Wild-type E. coli cells are sensitive to nitrofurazone (NF) and many other nitrofuran derivatives. A variety of evidence indicated that these compounds are converted to toxic active metabolites by reductases present in the bacteria. Sensitive E. coli K-12 acquired 3-fold greater resistance to NF in 1 mutational step. These partially resistant mutants could undergo a 2nd mutation that made them 10 times as resistant as the wild-type. Mutation of wild-type strain K-12 to the higher level of resistance in a single step was not observed. The 1st mutational step was associated with partial loss of reduced NADP-linked, O2-insensitive NF reductase activity, and the 2nd step was associated with loss of the remaining activity. The 2-step mutants contained other NF reductases that were inhibited by O2 and reduced NF only under anaerobic conditions. The genes that control reductase activity were designated as nitrofuran sensitivity genes (nfsA and nfsB). Thus, wild-type strains are nfsA+ nfsB+, and the resistant double mutants are nfsA nfsB. A variety of crosses established that these genes are both located close to gal, that the most probable sequence is lac nfsB gal nfsA, and that the single-step mutants with an intermediate level of resistance are nfsA nfsB+. The nfsA+ nfsB strains contained about 70-80% of the wild-type reductase I activity: apparently enough to confer wild-type sensitivity. This reductase activity was resistant to 2 M urea. The nfsA nfsB+ strains had only 20-30% of the wild-type activity, and this residual activity was sensitive to 2 M urea.