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Brazilian journal of microbiology : [publication of the Brazilian Society for Microbiology]2025; 56(4); 2987-2996; doi: 10.1007/s42770-025-01783-x

Streptococcus equi subsp. equi isolated from horses in Southern Brazil: molecular and phenotypic analyses.

Abstract: Streptococcus equi subsp. equi (S. equi) is the etiological agent of strangles, a contagious equine disease characterized by lymph node abscess and respiratory complications. To clarify the epidemiology and virulence factors of isolates, this study demonstrated phenotypic and genotypic differences between S. equi obtained from nasal secretions and lymph node aspirates of clinical strangles cases. Additionally, circulating alleles were differentiated through sequencing of the 5' end of the seM gene. A total of 23 clinical isolates collected from horses with strangles over the past decade were analyzed for phenotypic characteristics such as colony morphology, sugar fermentation, capsule production, biofilm formation, and antimicrobial susceptibility, as well as genotypic features. The analysis revealed phenotypic variability, particularly differences in sugar metabolism and capsule expression associated with colony morphology. Most isolates exhibited weak biofilm formation and susceptibility to cephalothin, ceftiofur, and streptomycin, while resistance to tetracycline was most common. Sequencing of the N-terminal region of the seM gene identified four alleles: seM-115, seM-158, seM-270, and seM-271. Of these, only seM-115 had previously been reported in Rio Grande do Sul State (southern Brazil). Phylogenetic analysis showed distinct clustering patterns, especially among the newly detected alleles (seM-270 and seM-271). These findings highlight the importance of integrated phenotypic and genotypic analyses to understand the diversity and potential virulence of circulating S. equi strains.
© 2025. The Author(s) under exclusive licence to Sociedade Brasileira de Microbiologia.
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Publication Date: 2025-10-01 PubMed ID: 41032072PubMed Central: PMC12660551DOI: 10.1007/s42770-025-01783-xGoogle 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 study analyzed the molecular and phenotypic characteristics of Streptococcus equi subsp. equi isolates collected from horses diagnosed with strangles in Southern Brazil.
  • The research aimed to understand the diversity, virulence factors, and antimicrobial resistance profiles of different strains to better inform epidemiology and control measures.

Background

  • Pathogen: Streptococcus equi subsp. equi is the causative agent of strangles, a contagious disease in horses characterized by lymph node abscess formation and respiratory issues.
  • Clinical relevance: Strangles is a significant disease in equine populations due to its contagiousness and clinical severity.
  • Need for study: Variations in phenotypic traits and genetic makeup of circulating strains impact disease management, vaccine development, and treatment strategies.

Objectives

  • To characterize phenotypic differences among S. equi isolates from nasal secretions and lymph node aspirates in clinical cases.
  • To analyze genotypic variation by sequencing the 5′ end of the seM gene, a known virulence marker.
  • To determine antimicrobial susceptibility and resistance patterns to guide effective treatment.

Methodology

  • Sample collection: 23 clinical isolates were obtained from horses diagnosed with strangles over approximately 10 years in Southern Brazil’s Rio Grande do Sul state.
  • Phenotypic analyses:
    • Colony morphology observation.
    • Sugar fermentation profiling to detect metabolic differences.
    • Capsule production assessment, important for virulence.
    • Biofilm formation tests, relevant to bacterial persistence and infection severity.
    • Antimicrobial susceptibility testing focused on antibiotics such as cephalothin, ceftiofur, streptomycin, and tetracycline.
  • Genotypic analyses: Sequencing of the N-terminal (5′) region of the seM gene to identify allelic variations among isolates.
  • Phylogenetic analysis: Constructed to observe clustering patterns and compare newly identified alleles with known ones.

Key Findings

  • Phenotypic Variability:
    • Differences in sugar metabolism were linked to colony morphology variations, showing metabolic diversity among strains.
    • Capsule expression also varied, influencing potential virulence.
    • Most isolates showed weak biofilm formation, suggesting limited capacity for persistent colonization or survival.
  • Antimicrobial Susceptibility:
    • Isolates were generally susceptible to cephalothin, ceftiofur, and streptomycin.
    • Tetracycline resistance was the most frequently observed resistance trait, indicating potential treatment challenges.
  • Genotypic Diversity:
    • Four alleles of the seM gene were detected: seM-115, seM-158, seM-270, and seM-271.
    • Only allele seM-115 had been previously reported in the same geographical region, showing new alleles emerging or previously undetected.
    • Phylogenetic clustering showed distinct groups, notably for the newly detected alleles seM-270 and seM-271, indicating genetic divergence.

Significance and Implications

  • The combination of phenotypic and genotypic analyses provides a comprehensive understanding of S. equi diversity in Southern Brazil.
  • Detection of new seM gene alleles suggests ongoing evolution or introduction of new strains in the region, which could affect vaccine efficacy and disease control.
  • Antibiotic resistance findings highlight the need for regional antimicrobial stewardship and careful selection of treatment regimens.
  • Understanding phenotypic traits like capsule expression and biofilm formation helps predict the virulence and persistence potential of strains, aiding in risk assessment.
  • These results support the development of targeted diagnostics and inform epidemiological tracking of strangles outbreaks.

Cite This Article

APA
Seeger MG, de Vargas APC, Vogel FSF, Cargnelutti JF. (2025). Streptococcus equi subsp. equi isolated from horses in Southern Brazil: molecular and phenotypic analyses. Braz J Microbiol, 56(4), 2987-2996. https://doi.org/10.1007/s42770-025-01783-x

Publication

Brazilian journal of microbiology : [publication of the Brazilian Society for Microbiology]
ISSN: 1678-4405
NlmUniqueID: 101095924
Country: Brazil
Language: English
Volume: 56
Issue: 4
Pages: 2987-2996

Researcher Affiliations

Seeger, Marlane Geribone
  • Graduate Program in Veterinary Medicine, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil.
de Vargas, Agueda Palmira Castagna
  • Department of Preventive Veterinary Medicine, Center of Rural Sciences, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil.
Vogel, Fernanda Silveira Flores
  • Department of Preventive Veterinary Medicine, Center of Rural Sciences, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil. floresfefe14@gmail.com.
Cargnelutti, Juliana Felipetto
  • Department of Preventive Veterinary Medicine, Center of Rural Sciences, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil. jucargnelutti@gmail.com.

MeSH Terms

  • Horses
  • Animals
  • Brazil / epidemiology
  • Horse Diseases / microbiology
  • Horse Diseases / epidemiology
  • Phylogeny
  • Streptococcal Infections / veterinary
  • Streptococcal Infections / microbiology
  • Streptococcal Infections / epidemiology
  • Anti-Bacterial Agents / pharmacology
  • Phenotype
  • Genotype
  • Virulence Factors / genetics
  • Microbial Sensitivity Tests
  • Biofilms / growth & development
  • Streptococcus / isolation & purification
  • Streptococcus / genetics
  • Streptococcus / classification
  • Streptococcus / drug effects

Grant Funding

  • 001 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
  • 140599/2022-3 / Conselho Nacional de Desenvolvimento Científico e Tecnológico
  • 302142/2022-3 / Conselho Nacional de Desenvolvimento Científico e Tecnológico
  • 308991/2023-0 / Conselho Nacional de Desenvolvimento Científico e Tecnológico

Conflict of Interest Statement

Declarations. Conflict of interest: The authors declare that they have no known competing conflicts of interest.

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