Genetics of nitrofurazone resistance in Escherichia coli

Wild-type Escherichia 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 threefold-greater resistance to NF in one mutational step. These partially resistant mutants could undergo a second 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 first mutational step was associated with partial loss of reduced nicotinamide adenine dinucleotide phosphate-linked, O(2)-insensitive NF reductase activity, and the second step was associated with loss of the remaining activity. The two-step mutants did, however, contain other NF reductases that were inhibited by O(2) and reduced NF only under anaerobic conditions. We designated the genes that control reductase activity "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 to 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 to 30% of the wild-type activity, and this residual activity was sensitive to 2 M urea.