There are no studies investigating interactions between clarithromycin or azithromycin and rifampin or other commonly used antimicrobial agents against virulent isolates of Rhodococcus equi. In addition, there is no published data on the postantibiotic effects (PAEs) and pharmacodynamics properties of antimicrobial agents against R. equi. The objectives were to assess in vitro interactions, pharmacodynamics, and PAEs of 11 antimicrobial agents belonging to various antimicrobial classes against R. equi. Antimicrobial agents investigated (erythromycin, clarithromycin, azithromycin, rifampin, amikacin, gentamicin, enrofloxacin, vancomycin, imipenem, ceftiofur, and doxycycline) were selected based on in vitro activity against large numbers of isolates of R. equi and frequency of use in foals or humans infected with R. equi. Three virulent strains of R. equi were evaluated by time-kill curves and checkerboard assays, and the postantibiotic effect was measured at 5×MIC. Only amikacin, gentamicin, enrofloxacin, and vancomycin were bactericidal against R. equi. Combinations including a macrolide (erythromycin, clarithromycin, azithromycin) and either rifampin or doxycycline, and the combination doxycycline-rifampin were synergistic. Combinations containing amikacin and erythromycin, clarithromycin, azithromycin, or rifampin and the combination gentamicin-rifampin were antagonistic. The PAEs of rifampin, erythromycin, clarithromycin, vancomycin, and doxycycline were relatively long with median values ranging between 4.5 and 6.5h. Azithromycin, gentamicin, and imipenem had intermediate PAEs ranging between 3.3 and 3.5h. Amikacin, enrofloxacin, and ceftiofur had shorter PAEs ranging between 1.3 and 2.1h. Gentamicin, amikacin, enrofloxacin, and doxycycline exhibited concentration-dependent activity whereas erythromycin, clarithromycin, azithromycin, rifampin, ceftiofur, imipenem, and vancomycin exhibited time-dependent activity against R. equi.
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