Characterization of Corynebacterium uterequi isolates from mares in Brazil

The genus Corynebacterium encompasses versatile bacteria that act as natural microbiota or pathogens, causing infections in humans and animals, including equine reproductive disorders. Corynebacterium uterequi (C. uterequi) is a potential pathogen in mares, associated with reproductive disorders including endometritis, embryonic loss, and infertility, raising concerns for equine reproductive health. This study aimed to characterize C. uterequi isolates from mares in southern Brazil and deepen the understanding of this species. Phenotypic, molecular, and phylogenetic analyses were performed on four C. uterequi isolates. Antimicrobial susceptibility was tested for 36 drugs by disk diffusion and five by microdilution. Biofilm formation was assessed at different glucose concentrations, and minimum inhibitory concentration (MIC) was determined for penicillin, gentamicin, doxycycline, clindamycin, and sulfamethoxazole + trimethoprim. These values were then used to evaluate biofilm eradication at concentrations up to 8× MIC. All isolates were confirmed as C. uterequi through phylogenetic analysis, clustering in the same group as others Corynebacterium spp. The isolates showed inhibition zones above Clinical & Laboratory Standards Institute and Brazilian Committee on Antimicrobial Susceptibility Testing susceptibility breakpoints for related species and formed biofilms with varying adhesion strengths. Biofilm eradication was effective during development but ineffective against mature biofilms. C. uterequi displays significant pathogenic potential in equine reproductive infections due to its ability to form persistent biofilms. Further studies are essential to optimize treatment strategies for chronic uterine infections caused by this pathogen.