Scientists have pinpointed a promising new source of antibiotics: ants. They have found that some species – including leaf-cutter ants from the Amazon – use bacteria to defend their nests against invading fungi and microbes.

Chemicals excreted by the bacteria as part of this fight have been shown to have particularly powerful antibiotic effects and researchers are now preparing to test them in animals to determine their potential as medicines for humans.

Doctors say new antibiotics are urgently needed as superbug resistance to standard antimicrobial agents spreads. More than 700,000 people globally now die of drug-resistant infections each year, it is estimated – and some health officials say this figure could be even higher.

Last week, UN secretary general Ban Ki-moon, speaking at the first general assembly meeting on drug-resistant bacteria, said antimicrobial resistance was now a fundamental threat to global health.

This was reiterated by Professor Cameron Currie of the University of Wisconsin–Madison, one of the scientists involved in the ant research.

“Antibiotic resistance is a growing problem,” he said last week. “However, pinpointing new antibiotics using the standard technique of sampling soil for bacteria is tricky. On average, only one in a million strains proves promising. By contrast, we have uncovered a promising strain of bacteria for every 15 strains we have sampled from an ant’s nest.”

Only a very specific group of ants are proving useful in this work, however. These are species that farm fungi in tropical regions in North and South America.

“These ants forage for plant material, which they bring back to their nests and feed to a fungus,” said Professor Jon Clardy of Harvard Medical School. “The fungus breaks down the plant material and the ants feed on the fungus.”

The strategy evolved around 15 million years ago, and has proved highly successful. There are now more than 200 ant species that farm fungi. Most fungus-farming ants simply forage for bits of old leaf or grass on the ground, however. A few, like leaf-cutter ants, cut leaves from trees and bring them back in pieces to their nest. “Plants are hard to digest, but fungi are good decomposers and break down plant material so ants can feed easily,” said Ethan Van Arnam, also of Harvard Medical School.

However, scientists have recently discovered that these nests are sometimes attacked by hostile fungi. “They kill off both the nest and its farmed fungus,” said Clardy. “In turn, ants have developed defences revealed as white patches on their bodies. They look as if they had been dipped in powdered sugar. These patches are made of bacteria which the ant stores on its body. Crucially, these bacteria produce powerful antibiotic and antifungal agents.”

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In this way, ants nurture bacteria which in turn make antifungal and antibacterial agents that defend nests. More to the point, these bacteria are similar to the ones used by pharmaceutical companies to make antibiotics. A typical example is Apterostigma ants, whose bacterial strains have been isolated in Panama and brought back to Harvard by Van Arnam. Many show promising antibiotic activity, he told the Observer.

“The ants don’t always win,” added Clardy. “You occasionally come across nests that have been overcome by invading fungi. But it is clear ants and their bacteria put up a very good fight, one that has been going on for millions of years. The result has been the production of some very interesting antibiotics.”

Clardy said foreign bacteria also attacked the ant’s microbe defences. “The bacteria in the nests get a really good deal. They are protected and fed by ants. Other strains of bacteria want to take over that comfortable niche. It is the bacterial equivalent of Game of Thrones. Everyone is trying to kill off everyone else and get to the top. The result has been the development of some very powerful antibiotic weapons. These are the end products of an arms race that has been going on for 15 million years. Our trick is to isolate the best of these weapons and use them to make new antibiotics for humans.”