Detection of polymyxin-resistant Enterobacteriaceae from poultry farms in Brazil: continued mcr gene dissemination.

The emergence and persistence of plasmid-mediated polymyxin resistance in Brazilian poultry production pose a significant One Health challenge. Here, cloacal swabs from 202 broilers across four farms in the State of Rio de Janeiro yielded 125 Enterobacteriaceae isolates growing on polymyxin-EMB agar. Escherichia coli accounted for 99% of resistant isolates, with one Klebsiella pneumoniae. Multidrug resistance (MDR) was observed in 75% of polymyxin-resistant strains. PCR screening revealed mcr-1 and mcr-5 genes. Conjugation assays demonstrated horizontal transfer of mcr-1 plasmids (48.5–194 kb). MLST assigned key strains to ST10 and ST48, both within the high-risk CC10 lineage. These findings underscore the entrenched nature of polymyxin resistance despite regulatory bans, highlight the risk of zoonotic transmission of MDR determinants, and call for enhanced surveillance, biosecurity and alternative interventions to mitigate the spread of mobile polymyxin resistance in poultry environments.