Genetics of Nitrofurazone Resistance in Escherichia coli

Abstract

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.