Antibiotic suceptibility of Rhodococcus equi

Azithromycin, tildipirosin, gamithromycin, rifampicin, doxycycline, amikacin, and ceftiofur were selected for susceptibility testing. Antibiotics were dissolved in appropriate solvents and diluted in sterile distilled water to prepare stock solutions following the guidelines by Clinical and Laboratory Standards Institute (35). Minimum inhibitory concentrations of the fourteen isolates of R. equi were obtained by the microdilution broth technique (35). Serial two-fold dilutions of the antimicrobial agents were prepared from the stock solution with concentrations ranged from 0.03 to 128 mg/L for all antibiotics. Serial dilutions were prepared using cation-adjusted Mueller–Hinton broth with 2.5% defibrinated horse blood. Inocula were produced by diluting the overnight culture of R. equi in cation-adjusted Mueller–Hinton broth with a buffered saline solution to a density of 0.5 on McFarland Turbidity Scale. This solution was diluted 20-fold and inoculated into each microtiter well. Finally, plates were incubated at 36°C and observed at 24 h. Plates were run in an ELISA microplate reader (ELx808TM, Biotek® Instruments, Inc.), absorbance was measured at 490nm wave-length. Each read was setup using the Gen 5 Reader Control (Biotek® Instruments, Inc.), the data output was exported automatically to Excel for further statistics analysis.