Colistin resistance develops as antibiotics are overused in human medicine as well as agriculture. Subtherapeutic doses of antibiotics have been used for decades to promote animal growth and prevent disease, rather than simply to treat disease. It is estimated that approximately 12,000 tons of colistin is used in food production annually, across the globe, and expected to rise to 16,500 tons by 2021.
Bacterial resistance to colistin can develop vertically when the resistance-conferring mcr-1 gene is passed from one generation to the next, or horizontally when plasmids containing mcr-1 are transferred between species. Plasmid-mediated colistin transfer allows particularly rapid spread of resistance.