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

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.