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.
Copyright © 2015 Elsevier B.V. All rights reserved.
Publication Date: 2015-05-27 PubMed ID: 26051479DOI: 10.1016/j.vetmic.2015.05.019Google Scholar: Lookup
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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This research article deals with a study of the efficacy of 10 different antimicrobial drugs against the intracellular bacteria, Rhodococcus equi, within horse-derived white blood cells. The results indicate that enrofloxacin, gentamicin, and vancomycin were the most effective drugs in the experimental setup.
Objective and Methodology
- The research’s primary objective was to study the activity of 10 different antimicrobial drugs against Rhodococcus equi, a bacteria that can infect the cells of a host organism, in this case, horse-derived macrophages (a type of white blood cell).
- The antibiotics tested were erythromycin, clarithromycin, azithromycin, rifampin, ceftiofur, gentamicin, enrofloxacin, vancomycin, imipenem, and doxycycline. These were chosen due to their ability to achieve desired concentrations in plasma at clinically recommended doses for foals.
- The researchers infected macrophages with R. equi and exposed them to these antibiotics. They then observed the number of bacterial colonies formed after 48 hours.
Findings
- The number of R. equi colonies in untreated control wells were significantly higher than those treated with any of the antimicrobial agents, indicating that these drugs could effectively reduce bacterial population.
- Among the tested antibiotics, enrofloxacin, gentamicin, and vancomycin showed more activity than the others. These drugs resulted in significantly lower numbers of R. equi colonies in the monolayers.
- In contrast, monolayers treated with doxycycline had significantly higher numbers of R. equi colonies as compared to other treatment regimes.
- No statistically significant difference was observed in the numbers of R. equi colonies between the monolayers treated with each of the other antimicrobial agents.
Implications and Future Directions
- Although this study provides valuable insights into the in vitro behavior of various antibiotics against R. equi, additional research is needed before these findings can be translated to clinical settings.
- The study underlines the importance of testing antimicrobials against intracellular bacteria for a closer association with in vivo effectiveness.
Cite This Article
APA
Giguère S, Berghaus LJ, Lee EA.
(2015).
Activity of 10 antimicrobial agents against intracellular Rhodococcus equi.
Vet Microbiol, 178(3-4), 275-278.
https://doi.org/10.1016/j.vetmic.2015.05.019 Publication
Researcher Affiliations
- Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, United States. Electronic address: gigueres@uga.edu.
- Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, United States.
- Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville FL 32610, United States.
MeSH Terms
- Actinomycetales Infections / drug therapy
- Actinomycetales Infections / microbiology
- Actinomycetales Infections / veterinary
- Animals
- Anti-Infective Agents / pharmacology
- Cytoplasm / microbiology
- Horse Diseases / drug therapy
- Horse Diseases / microbiology
- Horses
- Macrophages / microbiology
- Microbial Sensitivity Tests / veterinary
- Rhodococcus equi / drug effects
Citations
This article has been cited 7 times.- Erol E, Scortti M, Fortner J, Patel M, Vázquez-Boland JA. Antimicrobial Resistance Spectrum Conferred by pRErm46 of Emerging Macrolide (Multidrug)-Resistant Rhodococcus equi. J Clin Microbiol 2021 Sep 20;59(10):e0114921.
- Mourenza Á, Collado C, Bravo-Santano N, Gil JA, Mateos LM, Letek M. The extracellular thioredoxin Etrx3 is required for macrophage infection in Rhodococcus equi. Vet Res 2020 Mar 10;51(1):38.
- Mourenza Á, Gil JA, Mateos LM, Letek M. A Novel Screening Strategy Reveals ROS-Generating Antimicrobials That Act Synergistically against the Intracellular Veterinary Pathogen Rhodococcus equi. Antioxidants (Basel) 2020 Jan 28;9(2).
- Álvarez-Narváez S, Berghaus LJ, Morris ERA, Willingham-Lane JM, Slovis NM, Giguere S, Cohen ND. A Common Practice of Widespread Antimicrobial Use in Horse Production Promotes Multi-Drug Resistance. Sci Rep 2020 Jan 22;10(1):911.
- Hardefeldt L, Thomas K, Page S, Norris J, Browning G, El Hage C, Stewart A, Gilkerson J, Muscatello G, Verwilghen D, van Galen G, Bauquier J, Cuming R, Reynolds B, Whittaker C, Wilkes E, Clulow J, Burden C, Begg L. Antimicrobial prescribing guidelines for horses in Australia. Aust Vet J 2025 Dec;103(12):781-889.
- Huguet A-S, Gourbeyre O, Bousquet-Mélou A, Ferran AA, Lallemand EA. Reassessment of extracellular and intracellular activity of macrolides, rifampicin, and doxycycline against Rhodococcus equi based on bacterial counts and microscopy. Microbiol Spectr 2025 Sep 2;13(9):e0120525.
- Huguet AS, Gourbeyre O, Bernand A, Philibert C, Bousquet-Melou A, Lallemand EA, Ferran AA. Comparative bactericidal activity of four macrolides alone and combined with rifampicin or doxycycline against Rhodococcus equi at concentrations achievable in foals. Front Pharmacol 2024;15:1458496.
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