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As a microbiologist, Massimiliano Marvasi has spent years studying how microbes have defeated us. Many pathogens have developed resistance to penicillin and other antimicrobial drugs and now public health experts warn of a global crisis in the treatment of infectious diseases.
Today, Dr. Marvasi, principal investigator at the University of Florence in Italy, finds solace in the study of ants.
About 240 species of ants grow in underground mushroom gardens. They protect their farms against pathogens with the help of powerful chemicals secreted by bacteria on their bodies. Unlike humans, ants do not face an antimicrobial resistance crisis.
In their article in Trends in Ecology and Evolution, Dr. Marvasi and her colleagues say that mushroom ants could serve as a model for drug development. It's not just because they have antimicrobials, it's the way they use their medications.
Ants growing mushrooms bring leaves or other debris to gardens in their nests, where certain types of fungi grow. Fungi – which can not thrive anywhere else – develop into a dense web that ants feed on their larvae.
But the crops also attract a parasitic fungus called Escovopsis. He attacks the gardens and starves the ants.
"It's a war between ants and pathogens for the same food," said Dr. Marvasi.
Ants have powerful allies in this war: bacteria that live on their chest. The worker ants cover the eggs of some strains of these bacteria. When an ant matures, it feeds on bacteria with secretions from the glands on the thorax.
Bacteria reimburse ants for this special care by making powerful antimicrobials that kill Escovopsis, protecting the gardens from destruction.
The fact that bacteria make antimicrobials is hardly surprising. The ones that doctors prescribe today have been mostly found in the soil, made by microbes. What is amazing is that the chemicals used by ants work so well.
Escovopsis has developed defenses against bacteria, producing compounds that inhibit their growth. And yet, ants still manage to control these pathogens.
Dr. Marvasi and his co-authors – Ayush Pathak of Imperial College London and Steve Kett of Middlesex University London – say that it would be nice if we take a closer look at the ants to discover the secrets of their success.
An important advantage is that the bacteria on the ants make several antimicrobials at a time. "It's an impressive chemical plant," said Dr. Marvasi.
Powerful evolutionary forces create this variety, said Sarah Worsley, senior research associate at the University of East Anglia in England, who did not participate in the new study.
When ants look for garden fertilizer, they pick up random bacteria from the soil. These are in fierce competition with resident microbes for the nutrients provided by the ant glands. Natural selection favors residents who make powerful antimicrobials that keep newcomers away.
"These antimicrobials are produced as a result of this war on the surface of the ant," said Dr. Worsley. "Ants compete and use these antimicrobials to maintain their mushroom gardens."
Researchers are also learning how new antimicrobials are evolving. "We are accumulating so much new information at the molecular level now," said Katrin Kellner, molecular ecologist at the University of Texas at Tyler.
Sometimes resident bacteria on ants take a gene from competitors. The protein produced by the new gene can then alter the form of an existing antimicrobial.
Other mutations can mix the genetic switches in the DNA of the bacteria. As a result, it can produce new antimicrobial compounds.
But Ulrich Mueller, an evolution biologist at the University of Texas at Austin, warned that scientists might not yet understand the ants enough to learn from their success.
Some antimicrobials that kill Escovopsis, for example, also seem to protect ants from their own infections. It is possible that their evolution has little to do with the protection of mushroom gardens.
"This dimension has been completely ignored," said Dr. Mueller.
Dr. Marvasi and his colleagues study the evolutionary competition between bacteria on ants and Escovopsis, opposing organisms to each other in Petri dishes.
They want to see if the combination of related antimicrobials is the secret of ant success. Mimicking this strategy could help keep our own potent antimicrobial drugs.
"The idea may be to have a main antibiotic, but in a mixture of similar antibiotics," said Dr. Marvasi.
Dr. Worsley and her colleagues are reproducing the evolution of the bacteria living on ants. Scientists are refining the bacterial genes that produce antimicrobials, hoping to uncover new compounds that could affect human disease rather than garden pests.
"We are shortening evolution by taking inspiration from these arms races of the past," she said.
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