Amphibian microbiome could be key to fighting deadly fungal disease

Scientists looking to stop a devastating disease which has pushed amphibians into extinction may have found a clue to stopping it.

Chytridiomycosis is a fungal disease which has been implicated in the extinction of over 90 amphibian species in the past 50 years, as well as causing declines in hundreds more.

Australian researchers found that frogs with a more diverse set of bacteria and fungi living on their body, collectively known as the microbiome, may be better able to resist the disease.

Dr Donald McKnight, who led the research, says, 'Why some species and populations are able to survive chytridiomycosis, and others are susceptible, is not entirely clear, but variations in microbiomes may play a key role.'

'In our study, lace-lid frogs showed the lowest ability to recover and were also the species with the fewest types of bacteria and fungi present on it. That matches what some other studies have found and suggests that diverse microbiomes may be important.'

Their findings also suggest that the presence of bacteria which inhibit chytridiomycosis is not itself enough to stop the disease.

'Lace-lids had high levels of anti-fungal bacteria that are thought to play a protective role against chytridiomycosis,' Donald adds. 'In fact, the individuals with the highest levels of chytridiomycosis also tended to be the individuals with the highest levels of anti-fungal bacteria. This was really surprising.'

The research, published in the journal Ecosphere, opens new avenues for scientists and conservationists to consider in their battle against chytridiomycosis.

What is chytridiomycosis?

Chytridiomycosis is a disease caused by the fungus Batrachochytrium dendrobatidis (Bd) which infects the skin of amphibians. As well as providing a protective barrier, amphibian skin also regulates water, mineral and oxygen exchange.

When infected by Bd, the skin of adults becomes disrupted as the fungus' spores spread from the deeper layers to the surface. The spores secrete proteins that break down connective tissue, limiting the ability of the host to regulate its body and leading to death.

It is thought to have originated in the Korean peninsula during the late nineteenth and early twentieth centuries, from where it spread around the world as international trade grew over the next one hundred years.

Bd is now considered to have a worldwide distribution and be present in almost every amphibian population. This spread has been assisted by the ability of the fungus' spores to persist in bodies of water and moist soil for several months until a suitable host arrives.

As a result, chytridiomycosis has been suggested as the cause of extinction for a range of species, including unique gastric-brooding frogs that incubated eggs and tadpoles in their stomachs.

The impact of the disease has spurred research to find a cure, but progress has been limited. While captive amphibians have been helped to survive by raising their body temperature and using anti-fungal agents, these methods are difficult to carry out in the wild.

Furthermore, these techniques do not eliminate the fungus or provide lasting immunity, allowing the amphibians to become infected by chytridiomycosis again in the future.

Instead, the researchers behind the recent study turned to the microbes amphibians already possess as potential allies in the fight against Bd.

How can the microbiome combat disease?

The microbiome refers to all of the microorganisms which inhabit a particular environment. As well as large areas such as the Amazon rainforest, it also covers the microbes that live within the bodies of plants and animals.

The human microbiome has been of particular interest as the result of studies linking changes in gut bacteria to the ability of the body to fight disease, digest food and remain healthy. The microbiome's abilities are related to the compounds secreted by individual microbes, the species it contains and how they interact. 

While studies into human microbiome have been conducted for over a decade, it is only more recently that similar investigations have begun into how the microbiome of other organisms affects their health.

In the recent study, swab samples were taken from the body surface of four Australian frog species. These were then analysed to detect the range of bacteria and fungi living on the amphibians.

The scientists found that each species had varying amounts of microbial diversity, with thousands of bacterial and fungal groups living on the different frogs. The exact microbe species and the way they interact have previously been linked to a greater ability to fight fungal diseases.

Species with lower microbiome diversity were associated with poorer recovery from Bd outbreaks, even if they contained significant amounts of chytridiomycosis-inhibiting bacteria. This is at odds with some previous studies, with the researchers having suggested some potential explanations for their findings.

'It may be that the presence of the fungus that causes chytridiomycosis actually causes anti-fungal bacteria to thrive,' Donald says, 'or there may be some other factor in the frogs' environment that favours both the bacteria and the pathogen.'

'It is also possible that only frogs with high levels of anti-fungal bacteria can survive high infection loads but confirming this will require further research.'

The unusual findings could also relate to a survivorship bias, because the researchers may only have found individuals with high levels of inhibitory bacteria as frogs with lower levels had already died. Frogs which responded better to Bd may also have inhibitory bacteria as yet unknown to science which could explain their survival.

In future, the research could assist in the development of probiotic remedies for amphibians to help their microbiome resist chytridiomycosis by providing a tailor-made selection of microbes for each species.