A hidden source of resistance to the latest antibiotics has been discovered in Australia

Despite the headline, this is not about newly invented medicines. Polymyxins have been known for a long time, but today they remain one of the last lines of defence against certain resistant infections.

The new gene has been named mcr-12. So far, it has not been detected in common pathogens affecting humans, so there is no immediate threat to patients. The discovery should be viewed as an early warning: potentially dangerous resistance genes may be lurking not only in hospitals and on farms, but also in the environment.

Details

The gene was discovered in the bacterium Pigmentiphaga litoralis, which was found in the bottom sediments of Crooked Creek in the Australian state of New South Wales. The watercourse is situated next to a coal-fired power station’s sludge pond and is contaminated with heavy metals.

The researchers sequenced the bacterium’s genome and found that mcr-12 is located on a plasmid — a separate circular DNA molecule. Bacteria can exchange certain plasmids, so the genes carried by them require particular attention. However, the authors have not yet demonstrated that the plasmid carrying mcr-12 is capable of transferring independently to bacteria that are harmful to humans.

To test the gene’s function, the scientists removed the plasmid containing it from the original bacterium. Following this, the minimum concentration of polymyxin B required to inhibit its growth fell by a factor of 32 — from 80 to 2.5 micrograms per millilitre. When mcr-12 was reintroduced, resistance was restored.

The gene alters the outer membrane of the bacterial cell. It adds a small chemical group to one of its components, which prevents the antibiotic from binding effectively to its target and thus prevents it from destroying the cell.

In the laboratory, the researchers also introduced mcr-12 into several other bacterial species. In Pseudomonas aeruginosa and Acinetobacter baumannii — pathogens responsible for severe hospital-acquired infections — the concentration of antibiotic required to inhibit growth approximately doubled. This confirms that the gene can function in certain pathogenic bacteria, but does not yet mean that it will render them completely resistant to treatment.

In Escherichia coli, Klebsiella and some other species tested, no significant increase in resistance was observed.

Why this is important

Polymyxins, including colistin, are used against certain Gram-negative bacteria that are resistant to many other drugs. The spread of genes that reduce the effectiveness of these antibiotics could further limit treatment options.

What makes mcr-12 distinctive is where it was discovered. Related genes had previously been found mainly in bacteria associated with patients, hospitals, foodstuffs and farm animals. The new gene was first identified in a wild-type bacterium from contaminated freshwater sediments in the Southern Hemisphere.

The same plasmid also carries genes conferring resistance to arsenic, copper, cadmium, zinc and cobalt. The authors therefore suggest that heavy-metal contamination may help such plasmids to persist even in places where antibiotics are rarely used. This remains a hypothesis for now: the study does not prove that metals were the specific cause of the emergence of mcr-12.

Background

The first transferable resistance gene in this family, mcr-1, was described in 2015 in Escherichia coli. The discovery caused concern, as the gene was located on a plasmid and could be transferred between bacteria.

Researchers later discovered other variants of mcr. Most of these were found in settings where antibiotics are used particularly extensively: in healthcare settings and livestock farming. Much less is known about natural reservoirs of such genes.

A new study refines this picture. Resistance can develop and persist in complex natural communities not directly linked to the treatment of humans or animals. This is precisely why the authors are calling for increased monitoring of bacteria in water bodies, soils and contaminated industrial areas.

Source

The study was conducted by Brody F. Gilliet, Ram P. Mahardjan, Amy Kane and their colleagues from Macquarie University and the University of Sydney.

The study, ‘Novel polymyxin resistance gene family mcr-12 from environmental Pigmentiphaga litoralis’, was published in the journal *Nature Communications* in 2026.