The Twelfth Key to Our Last-Resort Antibiotic Turned Up in Freshwater Mud

Australian sediment near a coal ash basin is not where you'd expect to find a new way to defeat medicine's final line of defense. But there it was: a yellow-pigmented bacterium sitting in heavy metal-contaminated freshwater muck south of Sydney, carrying a gene no one had catalogued before, and that gene does something we needed it not to do.

Researchers from Macquarie University and the University of Sydney have identified mcr-12, a new member of the mobile colistin resistance gene family. The mcr genes are the ones that keep resistance scientists up at night: they're transferable, meaning bacteria can pass them to each other, and they confer resistance to polymyxins, the antibiotics reserved for infections that have beaten everything else. There are now 12 known mcr families. The previous 11 were found in clinical, food or livestock settings, in a class of bacteria called Gammaproteobacteria. Mcr-12 breaks both of those patterns at once. It came from environmental sediment, and it lives in a Betaproteobacterium called Pigmentiphaga litoralis that had no known business carrying antibiotic resistance genes.

The gene works. When researchers removed the plasmid carrying mcr-12, the bacterium became 32 times more susceptible to polymyxin B. When they reintroduced it, resistance returned. And when they transferred mcr-12 into Pseudomonas aeruginosa and Acinetobacter baumannii, two of the most feared drug-resistant hospital pathogens, it conferred polymyxin resistance in those species as well.

The gene appears to have been maintained in the sediment not because of antibiotic pressure, but because of heavy metal contamination. Resistance to arsenic and cadmium rode along on the same plasmid, keeping mcr-12 in circulation.

No one has found this gene in a hospital yet. That does not appear to be a permanent condition.

Read the full story at Nature Communications, July 16, 2026

Hot Take: The unsettling part isn't that bacteria found another way around last-resort drugs. It's that humans may have been maintaining the library where those instructions were stored.