Determination of the Mutant Selection Window and Evaluation of the Killing of <i>Mycoplasma gallisepticum</i> by Danofloxacin, Doxycycline, Tilmicosin, Tylvalosin and Valnemulin

<div><p><i>Mycoplasma gallisepticum</i> is a common etiological cause of a chronic respiratory disease in chickens; its increasing antimicrobial resistance compromises the use of tetracyclines, macrolides and quinolones in the farm environment. Mutant selection window (MSW) determination was used to investigate the propensity for future resistance induction by danofloxacin, doxycycline, tilmicosin, tylvalosin and valnemulin. Killing of <i>M</i>. <i>gallisepticum</i> strain S6 by these antimicrobials was also studied by incubating <i>M</i>. <i>gallisepticum</i> into medium containing the compounds at the minimal concentration that inhibits colony formation by 99% (MIC₉₉) and the mutant prevention concentration (MPC). Based on the morphology and colony numbers of <i>M</i>. <i>gallisepticum</i> on agar plates, the four kinds of sera in the order of the applicability for culturing <i>M</i>. <i>gallisepticum</i> were swine serum > horse serum > bovine serum > mixed serum. The MPC/MIC₉₉ values for each agent were as follows: danofloxacin > tilmicosin > tylvalosin > doxycycline > valnemulin. MPC generated more rapid and greater magnitude killing than MIC₉₉ against <i>M</i>. <i>gallisepticum</i>. Under exposure of 10⁵–10⁹ CFU/mL at MPC drug levels, valnemulin had the slowest rate of reduction in viable organisms and danofloxacin had the highest rate of reduction.</p>