Positive R plasmid mutator effect on chromosomal mutation to nalidixic acid resistance in nalidixic acid-exposed cultures of Escherichia coli.

Mutation frequencies to nalidixic acid resistance (15 mg/L in nutrient agar) were determined for derivatives of Escherichia coli AB1157 carrying the mutator plasmids R46, R391 or pYD1, or the non-mutator plasmid RP4. Frequencies of mutation remained constant in cultures of AB1157(R46) growing exponentially in drug-free broth, at a level about 12-fold higher than in the strain without plasmid. Mutation frequencies in cultures of strains AB1157(R391) and AB1157(pYD1) were about three times greater than in the control, whereas plasmid RP4 had no effect on spontaneous mutation frequency to nalidixic acid resistance. Exposure of strain AB1157 to 6 mg/L nalidixic acid in nutrient broth killed 80% of cells after 4 h. This enriched the proportion of nalidixic acid-resistant cells present in the surviving cell population giving enhanced "apparent" mutation frequencies. These were further increased by cell division of resistant mutants in the nalidixic acid-containing medium. "Apparent" resistance mutation frequencies in nalidixic acid-exposed cultures of the R46-, R391- or pYD1-carrying derivatives were, at their peak, 447-, 53- and 38-fold higher than in the control, the strain without plasmid, or the RP4-containing strain, respectively. These data illustrate how mutator plasmids like R391 and pYD1, which mediate only small increases in spontaneous mutation, can contribute to the development of clinically-significant levels of quinolone resistance.