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A fungus that kills frogs must be tackled with a cocktail of bacteria, rather than a single bacterium that protects the body from harm, researchers have found.
Their research, published in Applied and Environmental Microbiology last month (27 March), shows that several bacterial probiotics — harmless, protective bacteria — found on the skin of frogs are necessary to fight off Batrachochytrium dendrobatidis, also known as the chytrid fungus, which is partly responsible for a global decline of amphibians.
The fungus harms biodiversity, as frogs are crucial to forest food chains, and reduces the number of amphibian species from which important medical treatments could be derived. At present, around a third of all amphibian species are under threat of extinction. The International Union for Conservation of Nature and Natural Resources says that Batrachochytrium dendrobatidis is the biggest killer of amphibians after habitat loss and pollution.
“The chemicals they secrete may end up being useful as human pharmaceuticals.”
Reid Harris, James Madison University
The paper’s authors isolated 56 strains of non-harmful bacteria from the skin of Belizean frogs, and tested them against different strains of the fungus in the lab.
“We already knew that some bacteria could inhibit the fungus and that some couldn’t,” says lead researcher Rachael Antwis. “But what this paper shows is that a given bacterium cannot inhibit different strains of the fungus equally.”
The chytrid fungus was discovered in 1998 and is thought to have spread around the world from frog species exported from Africa. It can cause frogs and other amphibians to die out almost as quickly as they are discovered. In Panama, for example, the fungus has wiped out almost half the amphibian species.
The team found that only a mixture of probiotics was effective in preventing fungal growth because of the several strains of fungus in the wild. “So far, we’ve been focusing on single species probiotics,” says Reid Harris, a biologist at James Madison University in the United States, who also researches probiotics for amphibians. He agrees the answer might be to introduce a mixture of bacteria.
The fungus is thought to kill amphibians by thickening their outer skin layer, a process that changes how they regulate electronic impulses on their skin. It is not yet clear how the probiotic cocktail would be given to the frogs, but suggestions from the researchers include putting it into the soil or water where the animals live.
Changes in the presence of frogs have knock-on effects for mammals, birds and other reptiles, but also for insects and crops. The consequences of losing amphibian species, however, go beyond the ecosystem, Harris says, as they are of cultural importance to many indigenous communities. In addition, frogs and salamanders are the source of many substances with medical properties.
“The chemicals they secrete may end up being useful as human pharmaceuticals,” he says. “For example, some species secrete an antimicrobial peptide which has been shown to be anti-HIV.”
 Rachael E. Antwis and others Amphibian symbiotic bacteria do not show universal ability to inhibit growth of the global pandemic lineage of Batrachochytrium dendrobatidis (Applied and Enviromental Microbiology, 27 March 2015)
 Andrew J. Crawford and others Epidemic disease decimates amphibian abundance, species diversity, and evolutionary history in the highlands of central Panama (Proceedings of the National Academy of Sciences, August 2010)
 Simon N. Stuart and others Status and trends of amphibian declines and extinctions worldwide (Science, 14 October 2004)