4-Quinolone resistance mutations in the DNA gyrase of Escherichia coli clinical isolates identified by using the polymerase chain reaction.

Seven nalidixic acid-resistant clinical isolates of Escherichia coli were shown to carry resistance mutations in their gyrase A proteins. Six had serine-83 to leucine or tryptophan changes; the seventh had an aspartate-87 to valine substitution. The frequent occurrence of a mutation at serine-83 implies a key role for this residue in quinolone action.