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Microbiology spectrum2025; 13(9); e0120525; doi: 10.1128/spectrum.01205-25

Reassessment of extracellular and intracellular activity of macrolides, rifampicin, and doxycycline against Rhodococcus equi based on bacterial counts and microscopy.

Abstract: is an opportunistic intracellular pathogen responsible for severe respiratory infections in foals, requiring prolonged antibiotic treatment. Despite its susceptibility to multiple antibiotics, the eradication of intracellular remains challenging due to its ability to persist and survive within alveolar macrophages. This study aimed to assess the extracellular and intracellular activity of rifampicin, doxycycline, and four macrolides against while assessing current methodologies used to differentiate intracellular and extracellular bacteria. We found that the removal of extracellular with short times of exposure to bactericidal drugs was challenging. We thus proposed an alternative approach with washing steps to assess intracellular antibiotic activity by combining bacterial counts and optical microscopy. Our results indicated that clarithromycin alone, as well as azithromycin or gamithromycin in combination with doxycycline, dramatically reduced both intracellular and extracellular . This study also highlighted the need for refined methodologies to assess antibiotic efficacy against ultimately contributing to the development of optimized treatment strategies for infections in foals. Objective: is a major cause of pneumonia in foals and represents a serious challenge for the equine sector due to the need for prolonged and sometimes ineffective antibiotic treatments. This study highlights the limitations of current methods used to evaluate antibiotic activity against , by demonstrating that widely used bactericidal antibiotics like gentamicin may not eliminate extracellular bacteria as effectively as previously assumed, and we emphasize the risk of overestimating intracellular bacterial survival in experimental models. We therefore propose a new method that allows more accurate assessment of antibiotic efficacy within host cells. Our findings help refine testing strategies and support the identification of antibiotic combinations with effective intracellular activity, offering new perspectives for improving treatment protocols against infections in foals.
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Publication Date: 2025-07-29 PubMed ID: 40728410PubMed Central: PMC12403593DOI: 10.1128/spectrum.01205-25Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This research investigates how effectively various antibiotics work against Rhodococcus equi, a pathogen causing severe lung infections in foals.
  • The study evaluates both the bacteria living outside cells (extracellular) and inside immune cells (intracellular), proposing improved methods to assess antibiotic efficacy more accurately.

Background and Goals

  • Pathogen and Disease Impact: Rhodococcus equi is an opportunistic intracellular pathogen primarily responsible for serious respiratory infections in foals, which require extended antibiotic treatment.
  • Challenges: Despite the availability of multiple antibiotics that target R. equi, eradicating bacteria that survive inside alveolar macrophages remains difficult due to their persistence.
  • Study Aims:
    • To reassess the activity of rifampicin, doxycycline, and four macrolide antibiotics (including clarithromycin, azithromycin, gamithromycin) against R. equi.
    • To critically evaluate currently used methodologies that differentiate between intracellular and extracellular bacterial populations.

Methodological Insights and Innovations

  • Current Method Limitations:
    • Removing extracellular bacteria using short exposure to bactericidal antibiotics (like gentamicin) is challenging and may not be as effective as previously thought.
    • Such incomplete removal risks an overestimation of intracellular bacteria survival during experiments.
  • Proposed Alternative:
    • The study introduces a washing procedure designed to improve the removal of extracellular bacteria.
    • Combining quantitative bacterial counts with optical microscopy provides a more reliable way to assess intracellular antibiotic activity.

Key Findings on Antibiotic Activity

  • Macrolides: Clarithromycin alone was found to effectively reduce both intracellular and extracellular R. equi populations.
  • Combination Therapies: Combinations of azithromycin or gamithromycin with doxycycline showed significant reductions in intracellular and extracellular bacteria, indicating synergistic activity.
  • Implications for Treatment: These findings highlight potential antibiotic regimens more capable of targeting R. equi inside host cells, which could improve therapeutic outcomes in foals.

Significance for Research and Veterinary Medicine

  • Refinement of Testing Approaches: This work stresses the necessity for more precise experimental methods to distinguish bacterial localization and antibiotic effects, which could influence the interpretation of intracellular survival and drug efficacy.
  • Impact on Antibiotic Development: By identifying more effective combinations and assessing their intracellular activity accurately, the study contributes to optimizing antibiotic strategies specific to R. equi infections.
  • Broader Implications: Improved methodologies and treatment insights have the potential to reduce prolonged antibiotic use, mitigate resistance development, and improve recovery rates for foals suffering from pneumonia caused by R. equi.

Cite This Article

APA
Huguet A-S, Gourbeyre O, Bousquet-Mélou A, Ferran AA, Lallemand EA. (2025). Reassessment of extracellular and intracellular activity of macrolides, rifampicin, and doxycycline against Rhodococcus equi based on bacterial counts and microscopy. Microbiol Spectr, 13(9), e0120525. https://doi.org/10.1128/spectrum.01205-25

Publication

Microbiology spectrum
ISSN: 2165-0497
NlmUniqueID: 101634614
Country: United States
Language: English
Volume: 13
Issue: 9
Pages: e0120525
PII: e01205-25

Researcher Affiliations

Huguet, Anne-Sophie
  • INTHERES, Université de Toulouse, INRAE, ENVT, Toulouse, France.
Gourbeyre, Ophélie
  • INTHERES, Université de Toulouse, INRAE, ENVT, Toulouse, France.
Bousquet-Mélou, Alain
  • INTHERES, Université de Toulouse, INRAE, ENVT, Toulouse, France.
Ferran, Aude A
  • INTHERES, Université de Toulouse, INRAE, ENVT, Toulouse, France.
Lallemand, Elodie A
  • INTHERES, Université de Toulouse, INRAE, ENVT, Toulouse, France.

MeSH Terms

  • Rhodococcus equi / drug effects
  • Anti-Bacterial Agents / pharmacology
  • Animals
  • Horses
  • Rifampin / pharmacology
  • Macrolides / pharmacology
  • Doxycycline / pharmacology
  • Actinomycetales Infections / veterinary
  • Actinomycetales Infections / microbiology
  • Actinomycetales Infections / drug therapy
  • Horse Diseases / microbiology
  • Horse Diseases / drug therapy
  • Microbial Sensitivity Tests
  • Bacterial Load
  • Microscopy

Conflict of Interest Statement

The authors declare no conflict of interest.

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