June 8, 2008
Bacteria May Stop Infection and Death of Amphibians
One third of amphibians currently face extinction. This massive global decline in their population is largely due the chytrid fungus which is causing disease and killing many species. But according to recent findings, conservationists may be able to beat the fungus with bacteria.
Certain types of "friendly" bacteria which naturally live on amphibians may be able to disable the bacteria with the chemicals they produce. This bacterium, Janthinobacterium lividum, may help the frogs survive infection.
The waterborne attacking fungus, Batrachochytrium dendrobatidis, has destroyed many amphibians over the past decade and left conservationists frantically searching for a way to halt its seemingly unstoppable spread. Yet somehow, certain communities of some species have been able to withstand infection. Professor Reid Harris and his group of researchers at James Madison University in Harrisonburg, Virginia, have narrowed down possibilities, and it seems like those with Janthinobacterium lividumon on their skin may be the survivors.
Following some experimentation researchers realized that the red-backed salamander, which had a high concentration of the anti-chytrid metabolites on its skin, was able to kill off the chytrid with chemicals. According to Harris, "One of our hypotheses is that the bacteria live in some kind of defensive symbiosis with the frogs and salamanders."
The mountain yellow-legged frog of the Sierra Nevada Mountains is one of the species which is critically endangered due to the fungus. Only 20% of the species have survived over the past 15 years.
Harris and his colleagues treated the mountain yellow-legged frog with extra bacteria, which not only reduced the weight loss seen when the fungus attacked, but appeared to keep them alive longer on top of that. In Harris' words, "In the group we exposed to chytrid, about 50% to 60% have died. But of the ones where we added the bacterium (Janthinobacterium lividum) none have died, and we're about 140 days in now."
Harris and his team tend to wonder: if the bacteria are protective, why are they not present in large numbers around the colonies? The team has come up with several possibilities: habitat loss, pollution, climactic shifts. These might have reduced the bacterial cargo and opened the door to fungal attack. Scientists in Spain have confirmed one of these possibilities: rising temperatures may increase amphibians' vulnerability to the fungus.
Regardless of history, the recent discoveries mean that the bacteria might be of good use in defending against the chytrid fungus.
"It's tremendously exciting, because the other treatments for chytrid have problems," claimed Don Church, a scientist with Conservation International and senior director of the Amphibian Assessment Group which monitors trends worldwide.
"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," continues Church who advocates more research on amphibians who survive chytrid attack. "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."
If Harris' team continues to have positive results with the mountain yellow-legged frogs, they would like to start projects within the next few years in the wild.
"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.
"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."
The situation is so dismal that even taking a risk such as scattering bacteria in ponds and soil seems like a viable option. Only a few years ago amphibian specialists were claiming that the only hope for surviving species was to take them into captivity. These new findings give hope that captivity isn't the only option.
Even with a defense against the chytrid fungus, amphibians will remain threatened by habitat loss, climate change, pollution, viral disease, hunting, and introduce predators.
On the Net:
James Madison University
ASM 2008 meeting
Global Amphibian Assessment