Co-occurrence of mcr-1, mcr-4 and mcr-5 genes in multidrug-resistant ST10 Enterotoxigenic and Shiga toxin-producing Escherichia coli in Spain (2006-2017).

Colistin is an antimicrobial polypeptide commonly employed for controlling and treating neonatal and post-weaning diarrhoea (PWD) diseases caused by Enterotoxigenic and Shiga toxin-producing Escherichia coli (ETEC and STEC). The plasmid-mediated colistin resistance gene, mcr-1 was first described in late 2015 and, since then, multiple studies have reported its global distribution. In addition, five different mcr genes have been identified. The aim of this study was to characterise the colistin-resistant E. coli clonal groups implicated in PWD in farms of intensive pig production. Of 186 ETEC and STEC isolated in Spain from 2006 to 2017, 76.9% showed resistance to colistin. Of those, 102 were mcr-4 carriers, 37 mcr-1 and 5 mcr-5, with co-occurrence of mcr-1/mcr-4, mcr-1/mcr-5 and mcr-4/mcr-5 in five isolates. Three different mcr-4 variants were detected, including the new mcr-4.4 and mcr-4.5 described here. Interestingly, the clonal group ST10-A (CH11-24) appears to be primarily responsible for the spread of mcr-4. In summary, our results show that the pig industry is an important reservoir of colistin-resistant E. coli, carriers of other additional risk genes, such as blaESBL. These food-producing animals might be spreading a cocktail of multiple resistances, posing a worrisome threat to human health.