Colistin is an antibiotic of last resort, one of the final options left when all other drugs fail. It is an older antibiotic and sometimes toxic to the kidneys. Yet precisely because colistin is not a particularly safe drug and thus rarely used, bacteria didn’t develop resistance to it.
Until they did, of course. At first, the occasional resistance mutation popped up, here and there. Then in 2015, scientists surveilling Chinese pig farms reported the discovery of Escherichia coli bacteria with colistin resistance in a form that can spread with frightening ease. The resistance gene, which they called mcr-1, lived on a free-floating loop of DNA called a plasmid. Bacteria—even bacteria of different species—can swap plasmids back and forth. Just seven months later, another group in Belgium found a second, similar gene, mcr-2. And this week, the original group of scientists reported a third, this time more distinct, colistin-resistance gene, mcr-3, also on a plasmid and also found in E. coli from a Chinese pig farm. That plasmid where the researchers found mcr-3 also contained 18 other resistance genes against other antibiotics.
Researchers think they have another reason to worry about mcr-3. This resistance gene is very similar to a naturally occurring gene in Aeromonas, a type of bacteria ubiquitous in fresh and brackish waters. It could be that mcr-3 developed in Aeronomas in the first place, and though it’s not yet confirmed, these bacteria may be a reservoir of colistin resistance. “mcr-3 might exist everywhere,” says Yang Wang, a biologist at China Agricultural University and an author on the new report. Colistin resistance could be even more widespread than we thought.