The DNA gyrases from Mycobacterium avium, Mycobacterium smegmatis and Mycobacterium fortuitum by, peregrinum, which are species naturally resistant, moderately susceptible and susceptible to fluoroquinolones, respectively, were purified by affinity chromatography on novobiocin-Sepharose columns. The DNA gyrase inhibiting activities (IC50 values) of classical quinolones and fluoroquinolones were determined from the purified enzymes and were compared to the corresponding antibacterial activities (MICs). Regarding M. fortuitum by. peregrinum, which is nearly as susceptible as Escherichia coli, the corresponding MIC and IC50 values of quinolones were significantly lower than those found for M. avium and M. smegmatis (e.g. for ofloxacin, MICs of 0.25 versus 32 and 1 mu g ml(-1), and IC50 values of 1 versus 8 and 6 mu g ml(-1), respectively). Such a result could be related to the presence of Ser-83 in the quinolone-resistance-determining region of the gyrase A subunit of M. fortuitum by, peregrinum, as found in wild-type E. coli, instead of Ala-83 in M. avium and M. smegmatis, as found in fluoroquinolone-resistant E. coli mutants. The IC50 values of cluinolones against the M, avium and M. smegmatis DNA gyrases were similar, while the corresponding MICs were 32-fold higher for M. avium when compared to M. smegmatis, suggesting that an additional mechanism, such as a low cell wall permeability or a drug efflux, could contribute to the low antibacterial potency of quinolones against M. avium.
