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Background  The mutant prevention concentration (MPC) is an important parameter to evaluate the likelihood of growth of fluoroquinolone-resistant mutants for antimicrobial-pathogen combinations. The MPCs of fluoroquinolones for different canine pathogens have not been compared. In this study, we compared for the first time orbifloxacin MPCs between susceptible strains of  Escherichia coli ,  Pseudomonas aeruginosa , and  Staphylococcus pseudintermedius  of canine origin.    Methods  More than 10 10  CFU/ml of 10 strains of each bacterial species were inoculated onto Muller-Hinton agar supplemented with different concentrations of orbifloxacin from 1× to 64× minimum inhibitory concentration (MIC) and the MPCs were recorded. MICs of original strains and of mutants arising after exposure to sub-MPC concentrations (one per original strain) were determined in the presence or absence of efflux pump inhibitors (EPIs). The effects of quinolone resistance-determining region (QRDR) mutations were also examined.    Results  MPCs were significantly higher for  P. aeruginosa  (16–128 μg/ml) than for  E. coli  (0.5–32 μg/ml). MPCs for  S. pseudintermedius  varied between the low-susceptible (16–128 μg/ml) and the high-susceptible strains (4–16 μg/ml) and were the most broadly distributed among the three species. Regarding resistance mechanisms, only one QRDR mutation in  gyrA  was found in all of the 10 mutants of  E. coli  and in 4 of the 10 mutants of  P. aeruginosa , whereas mutations in both  grlA  and  gyrA  were found in 3 mutants and one mutation in  grlA  was found in 2 mutants among the 10 mutants of  S. pseudintermedius . In the presence of an EPI, the MICs of  P. aeruginosa  mutants decreased markedly, those of  E. coli  mutants decreased moderately, and those of  S. pseudintermedius  mutants were unaffected.    Conclusions  MPCs of orbifloxacin vary between bacterial species of canine pathogens, possibly due to the diversity of the main fluoroquinolone resistance mechanism among these species. Therefore, the type of bacterial species should be taken into consideration when using fluoroquinolone drugs such as orbifloxacin in canines.

acta veterinaria scandinavica

Mutant prevention concentration of orbifloxacin: comparison between Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus pseudintermedius of canine origin