Abstract
Enrofloxacin-resistant mutants of Stx2-producing Escherichia coli O157:H7 from cattle were selected. Mutants produced threefold higher Stx2 levels than native strains after induction with enrofloxacin. Mutants were also inducible using hundredfold higher enrofloxacin concentrations than the ones used for native strains. These results suggest that Escherichia coli O157:H7 from cattle may become more frequently pathogenic to humans as a side effect of the increasing use of veterinary fluoroquinolones.