Do ants hold key to drug resistance?

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Posted on Mon, Jan. 09, 2006

Do ants hold key to drug resistance?

BY SUSANNE RUST

Milwaukee Journal SentinelMILWAUKEE

http://www.sanluisobispo.com/mld/sanluisobispo/news/nation/13583355.h

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Some ants, it seems, are packing more than your picnic lunch.

According to researchers at the University of Wisconsin-Madison, a

particular tribe of ants, known as attine ants, have pockets

throughout their thick, outer armor crammed full of antibiotic-

producing bacteria. They use these bacteria to kill off a parasitic

fungus that could destroy their way of being.

And according to Cameron Currie, a University of Wisconsin

bacteriologist, the ants, the bacteria they cultivate and the fungus

they fear have been in a stalemate for millions of years. This

prompts the question: How come the fungus has not evolved to resist

this particular strain of bacteria? This question could trigger

insights into the battle against antibiotic-resistant strains of

disease.

The research was published last week in the journal Science.

Attine ants, which include New World leaf cutters, appear to depend

on a particular kind of fungus to live. They grow and cultivate

farms of friendly fungi, which provide them with nourishment and

shelter; the queen and larvae feed exclusively off the fungal garden.

The ants tend to the garden like good farmers: weeding, fertilizing

and providing nourishment to their fungal fields. In return, the

fungi benefits from the ants' meticulous care.

The relationship isn't unique. There are hundreds, if not thousands,

of examples across the living universe in which two different

organisms rely upon each other - something called obligate symbiosis

in the zoological nomenclature.

But Currie discovered that the attine ant-fungus association was

more complex than scientists thought.

The ants and their fungi live under constant siege from a parasitic

fungus called Escovopsis. This fungus can damage their gardens and

hamper, and sometimes destroy, their livelihood. But somehow the

ants are generally able to keep the bad fungus at bay.

Currie found that the ants carry a particular strain of antibiotic-

producing bacteria that is primed to kill the threatening fungus,

leading him to document one of the natural world's few cases

of " quadripartite symbiosis, " or an evolved system that includes

four interacting partners.

He said that even though these ants had been extensively studied for

more than century - with more than 3,000 scholarly papers published

on their behavior, ecology and biology - no one had realized what

they were looking at.

But, " it's pretty obvious once you see it, " said Currie, whose

research was conducted with an international team of scientists from

the University of Copenhagen, University of Texas and Catholic

University of Leuven in Belgium.

The bacterium, which is white and fiberlike, is conspicuous. But

like other observers of fungus-growing ants, Currie initially

dismissed the white fuzz on their bodies as " some sort of

dermatitis, you know, like a skin infection on humans. "

As the years clicked by, he noticed a pattern to the fuzzy white

blemishes and thought, " wait a second, this must be important. "

Indeed, he was finding it in every species of attine ant he studied,

but never on nonfungus-growing ants.

Then he noticed that those ants in the colony whose job it is to

tend the fungus garden had more of the white fuzz on their bodies

than others - so much so that he called these garden-tending

workers " sheep. "

" They're completely covered, " he said.

When he first published his results, in 1999, it was erroneously

reported that the bacterium was a kind of Streptomyces, a group of

bacteria well-known for producing human antibiotics.

This result was received with some skepticism, said Ulrich Mueller,

an ecologist at the University of Texas who was not involved in this

study.

Some claimed that Streptomyces was common, Mueller said, and

therefore not necessarily a unique product of this system. Currie

later discovered the bacterium was not of the Strep variety, but

instead one that isn't as widespread.

But, Currie said, whether the bacterium was Streptomyces or

Pseudonocardia - as he later discovered - was hardly a huge deal.

What is important in this work, he said, is showing that the ants

rely on a particular type of bacteria to fight the fungus.

Currie demonstrated that these ants have evolved to house and

cultivate bacteria by modifying their exoskeleton so that it is

perfused with large crypts - caverns that hold the bacteria, which

the ants presumably cultivate with specialized gland cells.

And, Currie said, each species of attine ant he has investigated has

its own crypt-morphology, or uniquely shaped physical caverns, to

house its specific strain of bacteria.

" This clinches the idea that these things must be absolutely

essential for these ants to function, " said Ted Schultz, chairman of

the department of entomology at the sonian Institution, who

wasn't part of the study. " They can't get along without it. "

He said Currie's research " opens a whole lot of questions we never

thought to ask. And clinches the idea that these bacteria have been

part of the system from the beginning. "

And this brings researchers to wonder how, if the system has been in

place for 50 million years, the antibiotic is still effective

against the fungus.

" Why is it that humans, who've been using antibiotics for 70 years,

are constantly having trouble with resistance evolution? " Schultz

said. " There probably is something we can learn from it. What is it

that changes in the bacteria, or the fungus, that we can zero in on? "

" If we understood this system better, " he said, we might learn

something that could benefit ourselves.