Inducible colistin resistance via a disrupted plasmid-borne mcr-1 gene in a 2008 Vietnamese Shigella sonnei isolate

The mcr-1 gene, which confers resistance against the last-resort antimicrobial colistin, was recently discovered in Enterobacteriaceae circulating in China. Through genome sequencing we identified a plasmid-associated inactive form of mcr-1 in a 2008 Vietnamese isolate of Shigella sonnei. The plasmid was conjugated into E. coli and mcr-1 was activated upon exposure to colistin, suggesting the gene has been circulating in human-restricted pathogens for some time but carries a selective fitness cost.