Relationship between danofloxacin PK/PD parameters and emergence and mechanism of resistance of Mycoplasma gallisepticum in In Vitro model

Mycoplasma gallisepticum is a serious pathogen for poultry that causes chronic respiratory disease in chickens. Increased embryonic mortality, as well as reduced weight gain and egg production have been found in infected chickens, which can lead to considerable economic losses in poultry production. Increased antibiotic resistance compromises the use of tetracyclines, macrolides and quinolones in the farm environment. In the present study, danofloxacin concentrations were simulated below the MIC(99), between the MIC(99) and MPC (the mutant prevention concentration), and above the MPC in an in vitro dynamic model against M. gallisepticum. The relationship between the simulated danofloxacin pharmacokinetics, pharmacodynamics (PK/PD) parameters and development of resistance for M. gallisepticum was explored based on the available data obtained from various dosing regimens in the in vitro model. Danofloxacin concentration, counts of viable cell and susceptibility were determined during the experiment. The mutations in gyrA, gyrB, parC and parE as well as efflux pumps were examined. The MIC of danofloxacin against M. gallisepticum was increased when drug concentrations were between the lower and upper boundaries of the mutant selection window. The upper boundary of the selection window in vitro was estimated as a C(max)/MPC value of 1. The lower boundary was estimated as C(max)/MPC value of 0.05. Both in terms of the MIC and resistance frequency, M. gallisepticum resistance was developed when danofloxacin concentrations fell inside the mutant selection window (ratios of C(max) to MPC between 0.05 and 1). The single mutation in gyrA (Ser-83→Arg) was found in all mutants, while double mutations in gyrA and parC (Ala-64→Ser) were observed only in the mutant with the highest MIC. In addition, no change of susceptibility in the mutants was observed in the presence of reserpine and carbonyl cyanide 3-chlorophenylhydrazone (CCCP). This suggested that ATP-binding cassette superfamily (ABC transporter) and major facilitator superfamily (MFS transporter) did not play a role in danofloxacin efflux.