Authors' reply: Escherichia coli harbouring mcr-1 gene isolated from poultry not exposed to polymyxins in Brazil.

To the editor: The foregoing letter by Lentz et al. examined the occurrence of the mcr-1 gene in Escherichia coli isolates recovered from chicken cloacal swabs collected between August and October of 2015, from a poultry slaughterhouse in southern Brazil [1], providing valuable additional data on the epidemiology of this novel gene. Of 343 animals screened, 10 (3%) different chickens belonging to three flocks from three different breeders were found with mcr-1 positive E. coli isolates [1]. None of these chickens had been reportedly exposed to polymyxins (as growth promoter) [1]. The authors therefore considered their findings as contradicting the plausible hypothesis that the emergence of the mcr-1 gene is linked to the use of polymyxins in animal feed in Brazilian livestock [2], suggesting that others compounds or factors may also be involved in the selection of this gene.