By Richard Black
Environment correspondent, BBC News website
Chytrid-infected frogs often show a characteristic hunched posture
The disease that is devastating amphibian populations around the world could be tackled using "friendly" bacteria, research suggests.
Scientists have found that certain types of bacteria which live naturally on amphibians produce chemicals that attack the disease-causing fungus.
Recent results indicate the bacteria help frogs survive fungal infection.
The chytrid fungus is a major reason for the global decline which sees one third of amphibians facing extinction.
But the latest findings, reported at the American Society for Microbiology meeting in Boston, may give conservationists a new way to tackle the scourge.
Reid Harris and colleagues found that treating the mountain yellow-legged frog (Rana muscosa) with extra helpings of bacteria reduces the weight loss seen when the fungus attacks, and appears to keep them alive longer as well.
"In the group we exposed to chytrid, about 50% to 60% have died," he told BBC News.
"But of the ones where we added the bacterium (Janthinobacterium lividum) none have died, and we're about 140 days in now."
The mountain yellow-legged frog of the Sierra Nevada mountains in the western US is categorised as Critically Endangered, with numbers believed to have fallen by 80% within about 15 years.
The waterborne fungus Batrachochytrium dendrobatidis has emerged as a major threat to amphibians in the last decade, and conservationists have been left grasping for a way of stopping its apparently inexorable worldwide spread.
WHAT ARE AMPHIBIANS?
First true amphibians evolved about 250m years ago
Adapted to many different aquatic and terrestrial habitats
Present today on every continent except Antarctica
Undergo metamorphosis, from larvae to adults
But although it has devastated many species, some appear to have an innate capacity to withstand infection. Even within species that generally succumb, the odd population survives.
What gives these communities immunity is not clear; but one answer, as Professor Harris's group has been finding, could be bacteria such as Janthinobacterium which live naturally on their skin.
Earlier lab experiments, also involving the red-backed salamander (Plethodon cinereus), showed that the bacteria produce chemicals able to attack the fungus.
"We detected anti-chytrid metabolites on the skin itself in high enough concentrations to kill off the chytrid," he said.
"One of our hypotheses is that the bacteria live in some kind of defensive symbiosis with the frogs and salamanders."
Another piece of evidence came with the finding that amphibians in colonies which survive the passage of the chytrid wave tend to carry higher levels of the bacteria.
This all raises questions as to why, if the bacteria are protective, they are not present in large enough numbers in all colonies; and whether some other factor - perhaps habitat loss, pollution or rapid climatic shifts - can reduce the bacterial cargo, opening up the door to fungal attack.
In Spain, scientists have found that rising temperatures appear to increase amphibians' vulnerability to infection.
Not in isolation
Whatever the history, the findings carry the promise that perhaps these bacteria could be used in the wild as a defence against the chytrid.
"It's tremendously exciting, because the other treatments for chytrid have problems," commented Don Church, a scientist with Conservation International and senior director of the Amphibian Assessment Group which monitors trends worldwide.
The JMU team applied protective bacteria to frogs in the lab
"The classical method of treatment with a fungicide leaves animals open to re-infection, and it's not a solution for use in the wild - it's a solution for animals that can be kept isolated or quarantined.
"So I think this is definitely a line of research that could become a tool applied to saving species in the wild, but we would have to develop a whole set of criteria for deciding where and how to use it - we have had so many catastrophes in the past through introducing species, so we have to be very careful."
Dr Church advocates more research on amphibians that survive chytrid attack, in order to catalogue what other varieties of defensive bacteria exist.
Reid Harris's team at James Madison University in Harrisonburg, Virginia, will continue to follow their treated mountain yellow-legged frogs to confirm that bacterial treatment really does keep them alive longer.
If the positive findings continue, they would like to start projects in the wild within a few years.
"Interestingly, some of the probiotic agricultural products that you can buy from hardware stores contain pretty similar bacteria to what we're using," he said.
The last few Panamanian golden frogs were taken into protective captivity
"Using them doesn't seem too controversial in an agricultural setting, although of course people get a lot more cautious when you're talking about national parks and so on.
"In something like Rana muscosa where the frogs pretty much stay put in ponds all year you might be able to add bacteria to soil or ponds and stay in front of the infection wave. It's harder to see how it would work in a tropical rainforest."
Scattering bacteria in ponds and soil might seem like a risky strategy.
But so dire is the chytrid situation that a few years ago, amphibian specialists were saying that the only solution for some species was to take the few remaining specimens into captive breeding programmes in the hope, rather than the certainty, that they could be re-introduced to the wild at some point in the future.
Having said that, a defence against chytridiomycosis would not by itself arrest the striking decline in amphibians, which are also threatened by habitat loss, pollution, climate change, viral disease, hunting and introduced predators.