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Amphibian Bacteria Fights Off Deadly Fungus, Study Says

Scott Norris
for National Geographic News
May 29, 2007
 
Bacteria that occur naturally on the skin of some salamanders can slow the progression of a deadly fungal disease responsible for mass die-offs of amphibians worldwide, a new study has found.

Because some amphibians do not carry the bacteria in sufficient amounts to battle the fungus, called chytrid, scientists took red-backed salamanders susceptible to the disease and bathed them in two separate types of bacteria.

Red-backed salamanders treated with laboratory-grown strains of the bacteria species Pedobacter cryoconitis, which had been isolated from disease-resistant individuals, were better able to fight off infection by the lethal fungus known as chytrid.

The finding strengthens the hope that biologists may be able to use the protective bacteria to inoculate some dwindling amphibian populations threatened by chytrid.

The chytrid epidemic has been particularly severe in portions of Central and South America and Australia.

The disease is ranked among the top perils facing amphibians—along with habitat destruction and climate change.

(See related: "Frog Extinctions Linked to Global Warming" [January 12, 2006].)

Reid Harris, of James Madison University in Harrisonburg, Virginia, delivered the new results last week at the annual meeting of the American Society for Microbiology in Toronto, Canada.

Harris's team had previously reported that eight different types of bacteria found in the skin secretions of two salamander species inhibited the growth of chytrid fungus in laboratory petri dishes.

The new work confirms that at least one bacteria species protects live amphibians, when administered at greater concentrations than may be present naturally.

Bacterial Armor

Many amphibian populations may naturally possess some anti-chytrid bacteria, Harris said, but not enough to keep the disease at bay.

In a separate study in California, his team showed that northern populations of the mountain yellow-legged frog are able to persist despite chytrid infection, while southern populations drop to near-extinction levels when the disease arrives.

In the frog populations that co-exist with chytrid, there are more animals with at least one species of anti-chytrid bacteria living on their skin, Harris said.

The ability to isolate, grow, and apply the beneficial bacteria may make it possible to bolster the defenses of amphibians living in areas where the arrival of the disease is expected.

One option is capturing amphibians at ponds and bathing them with anti-fungal bacteria, Harris said.

"The concept would be to build a 'fire line' to stop the epidemic spread of the pathogen," he said.

Karen Lips, of Southern Illinois University, in Carbondale, has been studying the effects of chytrid disease on frog populations in Central America.

The work by Harris's team is an "important discovery," Lips said, "because it provides additional evidence that some species of amphibians have defenses that can successfully defeat the disease.

"Just as we have used microbes to break down chemical contaminants and fight agricultural pests," she said, "we might be able to figure out how to use this microbe to protect frogs from chytrid in captivity, and perhaps in the wild."

Bacteria on Board

Other research has shown that some frogs produce chemical compounds that are effective defenses against chytrid. But this ability seems rare.

Work by Harris and others has also shown that communities of beneficial bacteria living in a frog's skin secretions assist in fighting off many types of fungal infection.

Bacterial defenses may be particularly vital for amphibians that care for their eggs, Harris said.

In such species, parent frogs and salamanders coat the developing embryos in mucus that is rich in antibiotic compounds produced by resident bacteria.

Harris's team was studying how bacteria help protect amphibian eggs from a different fungal disease when they came across the anti-chytrid strains.

The researchers are now working to isolate additional and perhaps more effective varieties of protective bacteria, focusing on amphibian species that have survived in regions where others perished from chytrid.

But Harris also added a note of caution, saying that the fungal diseases explosive spread may have been triggered by outside factors.

(See related: "Frog, Lizard Extinctions Caused by Climate, Not Fungus, Study Suggests" [April 17, 2007].)

Amphibians may have become more susceptible to the disease because their protective bacteria may have been damaged by global warming or pesticide contamination, Harris said.

If so, Harris added, "applications of beneficial skin bacteria may not be a long-term solution until underlying factors are addressed."

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