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Revista do Instituto de Medicina Tropical de Sao Paulo2025; 67; e3; doi: 10.1590/S1678-9946202567003

In vitro susceptibility pattern of Rhodococcus equi isolated from patients to antimicrobials recommended exclusively to humans, to domestic animals and to both.

Abstract: Rhodococcus equi is an opportunistic soil-borne bacterium that is eliminated in feces of multi-host animals. An increase in multidrug-resistant R. equi isolates has been reported in humans and domestic animals, and it has been hypothesized that the treatment of R. equi in foals could increase the selective pressure on multidrug-resistant isolates and favor human infections by resistant isolates. We investigated the in vitro antimicrobial susceptibility/resistance of 41 R. equi strains from humans, which were isolated from patients with pulmonary signs, using 19 antimicrobials from 10 distinct classes, recommended exclusively to humans, recommended exclusively to domestic animals and used in both. All isolates were subjected to mass spectrometry and identified as R. equi. Among the antimicrobials used exclusively in humans, tigecycline and vancomycin showed 100% efficacy. Amikacin, amoxicillin/clavulanic acid, imipenem, levofloxacin, clarithromycin, rifampin, ciprofloxacin, and gentamicin, used in both humans and animals, revealed high efficacy (97-100%). Conversely, a higher frequency of isolates was resistant to penicillin (87.8%) and trimethoprim/sulfamethoxazole (43.9%), which are used in both humans and animals. Among the antimicrobials used only in animals, isolates were resistant to florfenicol (46.4%), ceftiofur (17.1%), and enrofloxacin (2.5%). Multidrug resistance was observed in 34% of isolates. The identification of drug-resistant R. equi isolated from humans used exclusively in animals is circumstantial evidence of the pathogen transmission from domestic animals to humans. This study contributes to the molecular identification of Rhodococcus species from humans and to the epidemiological vigilance of the multidrug-resistant isolates.
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Publication Date: 2025-02-03 PubMed ID: 39907395PubMed Central: PMC11790073DOI: 10.1590/S1678-9946202567003Google 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.

This study tested 41 human clinical isolates of the bacterium Rhodococcus equi against 19 antibiotics and found that several human-only and shared human–animal drugs were highly active, while notable resistance existed to penicillin, trimethoprim-sulfamethoxazole, and certain veterinary-only antibiotics. The detection of resistance to antibiotics used exclusively in animals in human isolates supports concerns about cross-species transmission and highlights One Health antimicrobial stewardship needs.

What is Rhodococcus equi and why it matters

  • Rhodococcus equi is a soil-dwelling, opportunistic bacterium shed in the feces of multiple animal hosts and capable of causing pulmonary disease in humans, especially in immunocompromised individuals.
  • Because it circulates among animals and the environment, antibiotic use in veterinary settings (notably in foals) can shape resistance patterns that may spill over into human infections.
  • Rising multidrug resistance (MDR) in R. equi threatens effective empiric and targeted therapy in both veterinary and human medicine.

Study aim and design

  • Objective: Assess in vitro susceptibility of 41 R. equi strains isolated from humans with pulmonary signs to 19 antimicrobials from 10 classes, grouped by indication: human-only, animal-only, or used in both.
  • Identification: All isolates were confirmed as R. equi by mass spectrometry (e.g., MALDI-TOF), ensuring species-level accuracy.
  • Approach: Standardized susceptibility testing to quantify the proportion of isolates susceptible or resistant to each agent; focus on patterns relevant to One Health transmission dynamics.

Key susceptibility findings

  • Human-only antibiotics with 100% activity: tigecycline and vancomycin showed complete in vitro efficacy across all isolates tested.
  • Antibiotics used in both humans and animals with high activity (approximately 97–100% susceptible): amikacin, amoxicillin–clavulanic acid, imipenem, levofloxacin, clarithromycin, rifampin, ciprofloxacin, and gentamicin.
  • Interpretation: Multiple drug classes remain highly active in vitro, including glycopeptides (vancomycin), glycylcyclines (tigecycline), aminoglycosides, beta-lactam/beta-lactamase inhibitor, carbapenems, macrolides, rifamycins, and fluoroquinolones.

Notable resistance signals

  • Shared human–animal drugs with higher resistance: penicillin (87.8% resistant; roughly 36/41 isolates) and trimethoprim–sulfamethoxazole (43.9% resistant; roughly 18/41 isolates).
  • Animal-only drugs with resistance detected: florfenicol (46.4% resistant; roughly 19/41), ceftiofur (17.1%; roughly 7/41), and enrofloxacin (2.5%; roughly 1/41).
  • Multidrug resistance (MDR): observed in 34% of isolates (approximately 14/41), indicating concurrent resistance to multiple antimicrobial classes.
  • Implication: Resistance to animal-only antimicrobials in human isolates is a red flag for cross-species transmission or shared environmental reservoirs shaped by veterinary antibiotic use.

One Health and transmission implications

  • Finding resistance to veterinary-only drugs in human isolates provides circumstantial evidence that resistant R. equi strains selected in animal populations (e.g., treated foals) can reach humans via environmental or direct animal contact pathways.
  • This aligns with a One Health perspective: antibiotic practices in animals can influence resistance seen in human clinical pathogens, especially for environmental organisms like R. equi.
  • Enhanced surveillance linking human, veterinary, and environmental samples is needed to trace transmission chains and quantify risk.

Clinical relevance for human infection management

  • Therapeutic options supported by high in vitro activity include vancomycin, tigecycline, imipenem, aminoglycosides, fluoroquinolones, macrolides (clarithromycin), rifampin, and amoxicillin–clavulanate.
  • Drugs to avoid empirically based on high resistance in this cohort include penicillin and trimethoprim–sulfamethoxazole.
  • Combination therapy is often favored in severe or immunocompromised cases to reduce the risk of emergent resistance during treatment; susceptibility testing should guide regimen choice.
  • In vitro susceptibility does not guarantee clinical success; drug penetration into pulmonary lesions, intracellular activity, host immune status, and duration of therapy are critical considerations.

Public health and stewardship takeaways

  • Veterinary stewardship: Curbing unnecessary antimicrobial use in foals and other animals, especially with classes showing resistance signals (e.g., florfenicol, ceftiofur), may reduce selection of resistant strains with zoonotic potential.
  • Human stewardship: Use culture and susceptibility results to narrow therapy; avoid agents with predictable resistance (penicillin, TMP–SMX) unless testing supports use.
  • Policy and surveillance: Integrate antimicrobial resistance monitoring across human hospitals, veterinary clinics, and environmental settings to detect and respond to emerging MDR R. equi lineages.

Strengths of the study

  • Direct focus on human clinical isolates with pulmonary disease, providing immediately relevant susceptibility data for patient care.
  • Broad antibiotic panel spanning 10 classes and three usage categories (human-only, animal-only, shared), enabling nuanced interpretation of selection pressures.
  • Species confirmation by mass spectrometry enhances reliability of susceptibility results.

Limitations and cautions

  • In vitro design: Susceptibility results may not fully translate to clinical outcomes due to pharmacokinetics/pharmacodynamics and host factors.
  • Sample size and representativeness: 41 isolates provide valuable insight but may not capture geographic or temporal variability; the study setting and timeframe are not detailed in the abstract.
  • Genetic mechanisms: The study does not report genotyping or specific resistance determinants, limiting inference about clonal spread or plasmid-mediated resistance.
  • Causality: Evidence for animal-to-human transmission is circumstantial; environmental reservoirs and independent selection in humans could also contribute.

Future research directions

  • Genomic epidemiology to identify resistance genes, mobile elements, and potential transmission links between animal and human isolates.
  • Prospective, multi-center surveillance to track temporal trends in susceptibility and MDR phenotypes across regions.
  • Pharmacodynamic studies and clinical registries to correlate specific regimens with outcomes in pulmonary R. equi infection.
  • Assessment of stewardship interventions in veterinary settings to determine impact on human isolate resistance patterns.

Practical take-home points

  • Vancomycin and tigecycline were uniformly active; several shared-use agents (e.g., imipenem, aminoglycosides, macrolide, rifampin, fluoroquinolones) also showed high activity.
  • High resistance to penicillin and substantial resistance to TMP–SMX argue against their empiric use for suspected R. equi without susceptibility confirmation.
  • Detection of resistance to animal-only drugs in human isolates underscores One Health transmission concerns and the need for integrated stewardship.
  • MDR was present in one-third of isolates, reinforcing the importance of susceptibility-guided, often combination, therapy.

Cite This Article

APA
Ribeiro NG, Silva PD, de Lima Paz PJ, Arabe Filho MF, Listoni FP, Listoni EP, Panegossi LC, Ribeiro MG. (2025). In vitro susceptibility pattern of Rhodococcus equi isolated from patients to antimicrobials recommended exclusively to humans, to domestic animals and to both. Rev Inst Med Trop Sao Paulo, 67, e3. https://doi.org/10.1590/S1678-9946202567003

Publication

Revista do Instituto de Medicina Tropical de Sao Paulo
ISSN: 1678-9946
NlmUniqueID: 7507484
Country: Brazil
Language: English
Volume: 67
Pages: e3
PII: e3

Researcher Affiliations

Ribeiro, Nícolas Garcia
  • Fundação Educacional do Município de Assis, Faculdade de Medicina, Assis, São Paulo, Brazil.
Silva, Paulo da
  • Instituto Adolfo Lutz, Ribeirão Preto, São Paulo, Brazil.
de Lima Paz, Patrick Júnior
  • Universidade Estadual Paulista, Faculdade de Medicina Veterinária e Zootecnia, Departamento de Produção Animal e Medicina Veterinária Preventiva, Botucatu, São Paulo, Brazil.
Arabe Filho, Marcelo Fagali
  • Universidade Estadual Paulista, Faculdade de Medicina Veterinária e Zootecnia, Departamento de Produção Animal e Medicina Veterinária Preventiva, Botucatu, São Paulo, Brazil.
Listoni, Fernando Paganini
  • Universidade Estadual Paulista, Faculdade de Medicina Veterinária e Zootecnia, Departamento de Produção Animal e Medicina Veterinária Preventiva, Botucatu, São Paulo, Brazil.
Listoni, Evandro Paganini
  • Universidade Estadual Paulista, Faculdade de Medicina Veterinária e Zootecnia, Departamento de Produção Animal e Medicina Veterinária Preventiva, Botucatu, São Paulo, Brazil.
Panegossi, Letícia Colin
  • Universidade Estadual Paulista, Faculdade de Medicina Veterinária e Zootecnia, Departamento de Produção Animal e Medicina Veterinária Preventiva, Botucatu, São Paulo, Brazil.
Ribeiro, Márcio Garcia
  • Universidade Estadual Paulista, Faculdade de Medicina Veterinária e Zootecnia, Departamento de Produção Animal e Medicina Veterinária Preventiva, Botucatu, São Paulo, Brazil.

MeSH Terms

  • Rhodococcus equi / drug effects
  • Rhodococcus equi / isolation & purification
  • Animals
  • Anti-Bacterial Agents / pharmacology
  • Microbial Sensitivity Tests
  • Humans
  • Drug Resistance, Multiple, Bacterial
  • Actinomycetales Infections / microbiology
  • Actinomycetales Infections / veterinary
  • Animals, Domestic / microbiology

Conflict of Interest Statement

The authors declare no conflict of interests.

References

This article includes 40 references
  1. Majidzadeh M, Fatahi-Bafghi M. Current taxonomy of Rhodococcus species and their role in infections.. Europ J Clin Microbiol Infect Dis 2018;37:2045–2062.
    pubmed: 30159693
  2. Bryan LK, Alexander ER, Lawhon SD, Cohen ND. Detection of vapN in Rhodococcus equi isolates cultured from humans.. PloS One 2018;13:e0190829.
    pmc: PMC5754133pubmed: 29300774
  3. Ribeiro MG. Rhodococcus infection in animals. The Merck veterinary manual 12 th. Aiello SE, editor, editor. Duluth: Merck; 2022. [cited 2024 Dec 4]; pp. 1–13.
  4. Vazquez-Boland JA, Meijer WG. The pathogenic actinobacterium Rhodococcus equi: what's in a name?. Mol Microbiol 2019;112:1–15.
    pmc: PMC6852188pubmed: 31099908
  5. Haubenthal T, Hansen P, Kramer I, Gindt I, Jünger-leif A, Utermohlen O. Specific preadaptations of Rhodococcus equi cooperate with its virulence-associated protein A during macrophage infection.. Mol Microbiol 2023;119:285–301.
    pubmed: 36627747
  6. Salazar-Rodriguez D, Aleaga-Santiesteban Y, Iglesias E, Plascencia-Hernández A, Pérez-Gómez HR, Calderón EJ. Virulence plasmids of Rhodococcus equi isolates from Cuban patients with AIDS.. Front Vet Sci 2021;8.
    pmc: PMC7947234pubmed: 33718470
  7. MacArthur I, Anastasi E, Alvarez S, Scortti M, Vázquez-Boland A. Comparative genomics of Rhodococcus equi virulence plasmids indicates host-driven evolution of the vap pathogenicity island.. Gen Biol Evolut 2017;9:1241–1247.
    pmc: PMC5434932pubmed: 28369330
  8. Takai S, Sawada N, Nakayama Y, Ishizuka S, Nakagawa R, Kawashima G. Reinvestigation of the virulence of Rhodococcus equi isolates from patients with and without AIDS.. Lett Appl Microbiol 2020;71:679–683.
    pubmed: 32920889
  9. Suzuki Y, Kubota H, Madarame H, Takase F, Takahashi K, Sasaki Y. Pathogenicity and genomic features of vapN-harboring Rhodococcus equi isolated from human patients.. Intern J Med Microbiol 2021;311:e151519.
    pubmed: 34280738
  10. Val-Calvo J, Darcy J, Gibbons J, Creighton A, Egan C, Buckley T. International spread of multi-drug resistant Rhodococcus equi.. Emerg Infect Dis 2022;28:1899–1903.
    pmc: PMC9423909pubmed: 35997496
  11. Takai S, Tharavichitkul P, Takarn P, K hantawa B, Tamura M, Tsukamoto A. Molecular epidemiology of Rhodococcus equi of intermediate virulence isolated from patients with and without acquired immune deficiency syndrome in Chiang Mai, Thailand.. J Infect Dis 2003;188:1717–1723.
    pubmed: 14639543
  12. Stewart A, Sowden D, Caffery M, Bint M, Broom J. Rhodococcus equi infection: a diverse spectrum of disease.. IDCases 2019;15:e00487.
    pmc: PMC6329319pubmed: 30656137
  13. Ribeiro MG, Takai S, Vargas AC, Mattos-Guaraldi AL, Camello TC, Ohno R. Short report: identification of virulence associated plasmids in Rhodococcus equi in humans with and without acquired immunodeficiency syndrome in Brazil.. Am J Trop Med Hyg 2011;85:510–513.
    pmc: PMC3163875pubmed: 21896813
  14. Ribeiro MG, Lara GH, Silva P, Franco MM, Mattos-Guaraldi AL, Vargas AP. Novel bovine-associated pVAPN plasmid type in Rhodococcus equi identified from lymph nodes of slaughtered cattle and lungs of people living with HIV/AIDS.. Transbound Emerg Dis 2018;65:321–326.
    pubmed: 29226632
  15. Lin WV, Kruse RL, Yang K, Musher DM. Diagnosis and management of pulmonary infection due to Rhodococcus equi.. Clin Microbiol Infect 2019;25:310–315.
    pubmed: 29777923
  16. Jiang Y, Li J, Qin W, Gao XL, Zeng Y. Tuberculosis with cavities? Rapid diagnosis of Rhodococcus equi pulmonary infection with cavities by acid-fast staining: a case report.. Front Publ Health 2022;10.
    pmc: PMC9521651pubmed: 36187656
  17. Silva P, Silva JO. Rhodococcus equi: an approach to the methodology of presumptive identification and tests of sensitivity to antimicrobials.. Int J Health Sci 2023;3:1–18.
  18. Puig CR, Pilares L, Marco F, Vila J, Martínez-Martínez L, Navas J. Comparison of the Vitek MS and Bruker matrix-assisted laser desorption ionization time of flight mass spectrometry systems for identification of Rhodococcus equi and Dietzia spp.. J Clin Microbiol 2017;55:2255–2260.
    pmc: PMC5483928pubmed: 28490491
  19. Álvarez-Nárvaez S, Huber L, Giguère S, Hart KA, Berghaus RD, Sanchez S. Epidemiology and molecular basis of multidrug resistance in Rhodococcus equi.. Microbiol Mol Biol Rev 2021;85:e00011-21.
    pmc: PMC8139527pubmed: 33853933
  20. Zúñiga MP, Badillo E, Abalos P, Valencia ED, Marín P, Escudero E. Antimicrobial susceptibility of Rhodococcus equi strains isolated from foals in Chile.. World J Microbiol Biotechnol 2023;39.
    pmc: PMC10287571pubmed: 37347336
  21. Curtney H, Huber L. Rhodococcus equi: challenges do treat infections and to mitigate antimicrobial resistance.. J Equine Vet Sci 2023;127.
    pubmed: 37295760
  22. Willingham-Lane JM, Berghaus LJ, Berghaus RD, Hart KA, Giguère S. Effect of macrolide and rifampin resistance on the fitness of Rhodococcus equi.. Appl Environ Microbiol 2019;85:e02665-18.
    pmc: PMC6585491pubmed: 30683740
  23. Álvarez-Nárvaez S, Giguére S, Cohen N, Slovis N, Vazquez-Boland JA. Spread of multi-drug resistance Rhodococcus equi, United States.. Emerg Infect Dis 2021;27:529–537.
    pmc: PMC7853588pubmed: 33496218
  24. Sanz MG. Rhodococcus equi: what is new this decade?. Vet Clin North Am Equine Pract 2023;39:1–14.
    pubmed: 36898784
  25. Giguère S. Treatment of infections caused by Rhodococcus equi.. Vet Clin North Am Equine Pract 2017;33:67–85.
    pubmed: 28161038
  26. Bordin AI, Huber L, Sanz MG, Cohen ND. Rhodococcus equi foal pneumonia: update on epidemiology, immunity, treatment and prevention.. Equine Vet J 2022;54:481–494.
    pubmed: 35188690
  27. Quinn PJ, Markey BK, Leonard FC, Fitzpatrick ES, Fanning S, Hartigan PJ. Veterinary microbiology and microbial disease.. 2. Chichester: Wiley-Blackwell; 2011.
  28. Barreiro JR, Ferreira CR, Sanvido GB, Kostrzewa M, Maier T, Wegemann B. Short communication: identification of subclinical cow mastitis pathogens in milk by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.. J Dairy Sci 2010;93:5661–5667.
    pubmed: 21094737
  29. . Performance standards of antimicrobial susceptibility testing.. 33 th. Wayne: CLSI; 2023. [cited 2024 Dec 4]. https://clsi.org/media/tc4b1paf/m10033_samplepages-1.pdf edition..
  30. . Performance standards for antimicrobial disk and dilution susceptibility tests for bacteria isolated from animals.. 7 th edition. Wayne: CLSI; 2024..
  31. Giguère S, Prescott JF, Dowling PM. Antimicrobial therapy in veterinary medicine. 5 th. Ames: Wiley; 2013.
  32. Giguère S, Lee E, Williams E, Cohen ND, Chaffin MK, Halbert N. Determination of the prevalence of antimicrobial resistance to macrolide antimicrobials or rifampin in Rhodococcus equi isolates and treatment outcome in foals infected with antimicrobial-resistant isolates of R equi. J Am Vet Med Assoc 2010;237:74–81.
    pubmed: 20590498
  33. Silva P, Miyata M, Sato DN, Santos AC, Mendes NH, Leite CQ. Rhodococcus equi isolation from sputum of patients with suspected tuberculosis. Mem Inst Oswaldo Cruz 2010;105:199–202.
    pubmed: 20428681
  34. Berghaus LJ, Giguère S, Guldbech K, Warner E, Ugorji U, Berghaus RD. Comparison of Etest, disk diffusion, and broth macrodilution for in vitro susceptibility testing of Rhodococcus equi. J Clin Microbiol 2015;53:314–318.
    pmc: PMC4290945pubmed: 25378571
  35. Magiorakos AP, Srinivasan A, Carey RB, Carmeli Y, Falagas ME, Giske CG. Multidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance. Clin Microbiol Infect 2012;18:268–281.
    pubmed: 21793988
  36. Tamara S, den Boer MA, Heck AJ. High-resolution native mass spectrometry. Chem Rev 2022;122:7269–7326.
    pmc: PMC9052423pubmed: 34415162
  37. Golub B, Falk G, Spink WW. Lung abscesses due to Corynebacterium equi: report of first human infection. Ann Intern Med 1967;66:1174–1177.
    pubmed: 6067513
  38. Samies JH, Hathaway BN, Echols RM, Veazey JM, Pilon VA. Lung abscess due to Corynebacterium equi. Report of the first case in patient with acquired immune deficiency syndrome. Am J Med 1986;80:685–688.
    pubmed: 3963045
  39. Caterino-de-Araújo A, Los Santos-Fortuna E, Zandona-Meleiro MC, Calore EE, Perez Calore NM. Detection of the 20-kDa virulence-associated antigen of Rhodococcus equi in malakoplakia lesion in pleural tissue obtained from an AIDS patient. Pathol Res Pract 2000;196:321–327.
    pubmed: 10834389
  40. Rocha BZ, Portilho FV, Garino F, Júnior, Monti FS, Almeida BO, Souza AA. Cellulitis-related Rhodococcus equi in a cat harboring VAPA-type plasmid pattern. Microb Pathog 2021;160.
    pubmed: 34509529

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