Persistence of colistin-resistant Escherichia coli and the mcr-1 gene in the poultry chain, after the colistin ban.

The increase in colistin resistance in diverse environments, led to the ban on colistin use in veterinary medicine in Argentina. The present study aimed to assess the presence of colistin resistance and the mcr genes in E. coli isolated from Argentinian poultry slaughterhouses and retail markets, following the ban on colistin use. The multidrug resistance profiles, the presence of the stx1, stx2, and eae virulence genes, and the clonal relationship among isolates were also analysed. At the slaughterhouse, 549 samples were collected, including caecal content (n = 270), neck skin (n = 270), and slaughterhouse wastewater (n = 9). At the retail market, 241 breast skin samples were collected. The isolates were obtained from TBX agar supplemented with colistin, followed by determination of the uspA gene. Colistin agar test and agar microdilution were used to determine the minimum inhibitory concentration for colistin and other critical antibiotics. Multiplex polymerase chain reactions were used to determine the presence of mcr and virulence genes. Pulsed-field gel electrophoresis was used to compare different pulse types. At the slaughterhouse, colistin-resistant E. coli (n = 49) were detected in 36 samples. At retail market colistin-resistant E. coli (n = 2) were recovered from one sample (0.41%). Among colistin-resistant isolates (n = 51), 58.82% carried the mcr-1 gene, and 90.19% exhibited multidrug resistance. The highest proportion of colistin-resistant E. coli was grouped into 11 clusters. Stx1, stx2, and eae genes were identified in two isolates. These findings demonstrate the persistence and circulation of colistin-resistant E. coli and the mcr-1 gene in Argentinian slaughterhouses and retail markets after the colistin ban.