The enhancement of R-plasmid-mediated resistance to aminoglycoside antibiotics by mutations to nitrofurantoin resistance in Escherichia coli

Mutation to high level resistance to nitrofurantoin of a strain of Escherichia coli carrying an R-plasmid conferring resistance to gentamicin, kanamycin and other antibiotics resulted in greatly increased resistance to the two aminoglycosides but not the other antibiotics. A similar result was observed when the R-plasmid was transferred to high level nitrofurantoin-resistant mutants. The enhanced resistance was accompanied by a 30% increase in 3-N-acetyltransferase activity. Resistance to chloramphenicol in a strain carrying an R-plasmid coding for chloramphenicol acetyltransferase was not increased although the same strain showed increased resistance to kanamycin. These observations may have considerable significance in the development of resistance to gentamicin and other aminoglycoside antibiotics.