title: Activity of 10 antimicrobial agents against intracellular Rhodococcus equi.
abstract: Studies with facultative intracellular bacterial pathogens have shown that evaluation of the bactericidal activity of antimicrobial agents against intracellular bacteria is more closely associated with in vivo efficacy than traditional in vitro susceptibility testing. The objective of this study was to determine the relative activity of 10 antimicrobial agents against intracellular Rhodococcus equi. Equine monocyte-derived macrophages were infected with virulent R. equi and exposed to erythromycin, clarithromycin, azithromycin, rifampin, ceftiofur, gentamicin, enrofloxacin, vancomycin, imipenem, or doxycycline at concentrations achievable in plasma at clinically recommended dosages in foals. The number of intracellular R. equi was determined 48h after infection by counting colony forming units (CFUs). The number of R. equi CFUs in untreated control wells were significantly higher than those of monolayers treated with antimicrobial agents. Numbers of R. equi were significantly lower in monolayers treated with enrofloxacin followed by those treated with gentamicin, and vancomycin, when compared to monolayers treated with other antimicrobial agents. Numbers of R. equi in monolayers treated with doxycycline were significantly higher than those of monolayers treated with other antimicrobial agents. Differences in R. equi CFUs between monolayers treated with other antimicrobial agents were not statistically significant. Enrofloxacin, gentamicin, and vancomycin are the most active drugs in equine monocyte-derived macrophages infected with R. equi. Additional studies will be needed to determine if these findings correlate with in vivo efficacy.
authors.affiliation: Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, United States. Electronic address: gigueres@uga.edu.
authors.forename: Steeve
authors.initials: S
authors.lastname: Giguère
authors.affiliation: Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, United States.
authors.forename: Londa J
authors.initials: LJ
authors.lastname: Berghaus
authors.affiliation: Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville FL 32610, United States.
authors.forename: Elise A
authors.initials: EA
authors.lastname: Lee
mesh_terms: D000193:Actinomycetales Infections / Q000188:drug therapy / Q000382:microbiology / Q000662:veterinary*; D000818:Animals; D000890:Anti-Infective Agents / Q000494:pharmacology*; D003593:Cytoplasm / Q000382:microbiology; D006734:Horse Diseases / Q000188:drug therapy* / Q000382:microbiology; D006736:Horses; D008264:Macrophages / Q000382:microbiology; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D016929:Rhodococcus equi / Q000187:drug effects*
chemical_list: D000890:Anti-Infective Agents
keywords: Antimicrobial; Foal; Intracellular; Macrophage; Rhodococcus equi