First Pandemic blaCTX-M-8-Producing ST224 E. coli in Brazilian Sheep: Resistance and Genomic Traits

Shiga toxin-producing E. coli (STEC) strains are particularly concerning due to their zoonotic potential and environmental persistence. Ruminants, especially sheep and cattle, serve as primary reservoirs, often shedding pathogenic strains asymptomatically and contributing to foodborne outbreaks through contamination of animal-derived products. This study aimed to characterize antimicrobial-resistant E. coli strains isolated from dairy sheep in Brazil, focusing on phylogenetic backgrounds, resistance profiles, and genomic features. From 65 rectal swab samples collected across five herds in two Brazilian states, 65 E. coli isolates were recovered. Of these, 27.7% showed antimicrobial resistance to at least one drug tested, and 32.3% were identified as STEC. Resistance was most frequently observed against sulfamethoxazole-trimethoprim, tetracycline, and gentamicin. Notably, one isolate (LZB-RS-110) exhibited an extended-spectrum β-lactamase (ESBL) phenotype and a multidrug-resistant profile. Whole-genome sequencing identified clinically relevant resistance genes (e.g., bla_CTX-M-8, tetB, sul2), virulence genes (stx1, stx2), and metal tolerance operons. The bla_CTX-M-8 gene, harbored on a predicted conjugative IncI1 plasmid, was flanked by mobile genetic elements, suggesting a high potential for horizontal gene transfer. Phylogenomic analysis revealed that LZB-RS-110 is closely related to international isolates from wild and domestic animals, highlighting the global dissemination of high-risk E. coli lineages. These findings underscore the critical role of sheep in the ecology of zoonotic and resistant E. coli, and the broader implications for food safety and One Health. Enhanced surveillance and rational antimicrobial use in livestock are urgently needed to mitigate the spread of resistance and safeguard public health.