Development of Resistance to Ofloxacin

Summary

As a member of the 4-quinolone group of antibacterial agents, ofloxacin shares the almost unique feature of being exempt from plasmid-borne resistance in either Gram-negative or Gram-positive bacteria. In the light of this feature, the development of resistance mediated through chromosomal mutation has been carefully studied, particularly the processes of mutation which confer resistance to levels of ofloxacin approaching those obtained at the site of infection after oral administration.

With Escherichia coli strain KL16 being used as a model system, the genetics of the development of resistance to ofloxacin at least 2 mg/L have been studied. In common with many in vitro studies of the development of resistance to the newer 4-quinolones, it has been observed that the mutation frequency was extremely low (in the range 1 × 10⁻¹⁰ to 1 × 10⁻¹²) with bacteria grown under routine laboratory conditions. The resultant organisms were very slow growing, temperature sensitive and apparently auxotrophic. The mutation(s) were, however, very unstable and the mutants readily reverted to ofloxacin sensitivity in the absence of selection with ofloxacin. Subsequent studies of spontaneous mutation under growth conditions more closely related to the in vivo situation in the lumen of the gut, with limitation of oxygen supply, showed that mutation frequencies were in the order of 1 × 10⁻⁸. Mutants obtained under these conditions displayed the same phenotype as found previously and were equally unstable.

Examination of the physiology of the ofloxacin-resistant mutants has shown that they display significant metabolic defects with regard to being able to cope with environmental fluctuations. Evidence is presented to show that such ofloxacin-resistant mutants are essentially handicapped and unlikely to be able to compete with ofloxacin-sensitive organisms.

These data suggest that in vivo mutation frequencies to resistance to the newer 4-quinolones are related to the conditions pertaining at the site of colonisation, and that existing in vitro estimations represent significant underestimation of what is likely to occur in the clinical situation. However, mutants resistant to ofloxacin are unlikely to represent a serious clinical problem because of their handicapped physiology and the likelihood of reversion to ofloxacin sensitivity.