Killer Fungus

In the late 1970s, scientists began discovering grisly scenes in the ponds and streams around Queensland, Australia. Entire populations of frogs were found floating dead in the waterways or strewn belly-up on shorelines—with their skin ripped to shreds. Over the next decade, scientists realized that the same mysterious phenomenon was occuring all around the world. In the late 1990s, the cause was finally identified: the chytrid fungus. This waterborne microbe infects the skin of frogs and other amphibians and usually proves fatal. For years, scientists struggled to find a way to stop the fungus from spreading.

Then, in 2001, Reid Harris, a biologist at James Madison University in Virginia, was watching a female four-toed salamander wiggle through her gelatinous eggs. He and other scientists knew this behavior passed on beneficial probiotic bacteria from the mother’s skin to her babies. The tiny microbes protect the developing salamanders from infections. Harris thought: Maybe similar bacteria could protect adult amphibians from chytrid. If so, it could put an end to the infection plaguing amphibians in nearly every corner of the globe.

In the late 1970s, scientists began finding horrible scenes around Queensland, Australia. Entire populations of frogs lay dead in the ponds and streams. The frogs were floating in the waterways or belly-up on shorelines. Their skin was ripped to pieces. Over the next decade, scientists realized that the same mystery was happening all around the world. In the late 1990s, they finally found the cause: the chytrid fungus. This microbe is carried by water. It infects the skin of frogs and other amphibians and usually kills them. For years, scientists searched for a way to stop the fungus from spreading.

Then, in 2001, Reid Harris had an idea. He’s a biologist at James Madison University in Virginia. As he watched, a female four-toed salamander wiggled through her jelly-like eggs. He and other scientists knew why she did this. She was passing on helpful probiotic bacteria from her skin to her babies. The tiny microbes protect the developing salamanders from infections. Harris wondered if similar bacteria could protect adult amphibians from chytrid. If so, it could end the infection killing amphibians worldwide.

People often refer to the spread of chytrid fungus as “the amphibian apocalypse”—and for good reason. It is responsible for the decline of more than 500 species of frogs and salamanders, and it has already led to the extinction of at least 90. Scientists estimate that more than 4,000—or half of all amphibian species—could be wiped out if the fungus continues to spread.

A widespread loss of amphibians could have a big impact around the planet, says Harris. “Amphibians are a critical part of many ecosystems and food webs,” he says. “They prey on insects, controlling their populations, and they provide food for other animals, like snakes and mammals.”

People often call the spread of chytrid fungus “the amphibian apocalypse.” And they have good reason. Chytrid has caused the decline of more than 500 species of frogs and salamanders. And it has led to the extinction of at least 90. What if the fungus continues to spread? Scientists think that more than 4,000 species could disappear. That’s half of all amphibian species.

The loss of so many amphibians could have a big effect around the planet, says Harris. “Amphibians are a critical part of many ecosystems and food webs,” he says. “They prey on insects, controlling their populations, and they provide food for other animals, like snakes and mammals.”

There are two known types of chytrid fungi: Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). Bd most often infects frogs, while Bsal usually infects salamanders. Scientists believe that both strains originated on the Korean Peninsula. Because the native amphibians there evolved with the fungi, their disease-fighting immune systems are able to fend off chytrid.

During the Korean War in the 1950s, soldiers returning home may have brought amphibian pets with them that harbored the fungi. That may be how chytrid first spread to other parts of the world. Amphibians exposed to the fungi for the first time had no immunity against them.

Chytrid reproduces through spores (see Fungus Life Cycle). These spores can swim in fresh water from one amphibian to another. Then they latch onto a frog or a salamander. The fungus grows, spreading across the animal’s body. “It burrows all over the skin until it can’t function,” says Molly Bletz, a biologist at the University of Massachusetts, Boston. “It’s so damaging because amphibians breathe through their skin.” In most cases, the infection is a death sentence, killing the animal in as little as two weeks.

Two types of chytrid fungi are known. They’re called Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). Bd mostly infects frogs, and Bsal usually infects salamanders. Scientists believe that both types started on the Korean Peninsula. The amphibians that live there developed with the fungi. So their disease-fighting immune systems can fight off chytrid.

Soldiers returned home from the Korean War in the 1950s. They may have brought home amphibian pets that carried the fungi. Chytrid may have first spread to other lands in that way. Then amphibians got exposed to the fungi for the first time, and they had no defenses against them.

Chytrid multiplies through spores (see Fungus Life Cycle). These spores can swim in fresh water from one amphibian to another. Then they hang onto a frog or a salamander. The fungus grows and spreads across the animal’s body. “It burrows all over the skin until it can’t function,” says Molly Bletz, a biologist at the University of Massachusetts, Boston. “It’s so damaging because amphibians breathe through their skin.” The infection is usually a death sentence. It kills the animal in as little as two weeks.

Today, the chytrid fungus has been detected on every continent except Antarctica (see Chytrid Worldwide). To combat its spread, scientists have been looking for medicinal treatments. For example, researchers tried bathing infected frogs in different antifungal solutions. They worked, clearing the frogs’ skin of fungus. But the effect was short-lived. Once the antifungal solution washed off, the amphibians were reinfected in the wild.

That’s why Harris, who’d observed the salamander mother transferring probiotic bacteria to her eggs, thought beneficial microbes might provide a better solution. Good bacteria could become part of amphibians’ natural microbiome—the community of microorganisms that live in and on an organism’s body. This would provide long-term protection against chytrid. So Harris and his team began looking for bacteria with antifungal properties that were also found naturally on the skin of amphibians.

Today, the chytrid fungus has been found on every continent except Antarctica (see Chytrid Worldwide). Scientists have been looking for medicines to fight its spread. For example, researchers washed infected frogs in different antifungal solutions. They worked to clear the frogs’ skin of fungus. But the effect didn’t last long. The antifungal solution washed off. Then the amphibians were reinfected in the wild.

That’s why Harris was looking for another idea. He saw the salamander mother pass along probiotic bacteria to her eggs. And he thought helpful microbes might provide a better solution. Good bacteria could become part of amphibians’ natural microbiome. That’s the group of microorganisms living in and on an organism’s body. This would provide lasting protection against chytrid. So Harris and his team began looking for bacteria. It had to have antifungal properties and occur naturally on amphibians’ skin.

Harris swabbed the skin of frogs and transferred any bacteria to petri dishes containing nutrientrich agar jelly. He then cultured, or grew, the different microbes and tested their antifungal properties by exposing the dishes to Bd. “Some bacteria did nothing, and some even helped the fungus thrive,” says Harris. But one bacterium, called Janthinobacterium lividum, or J. liv, killed the fungus. It produces a purple chemical called violacein, a substance already known to have antifungal properties.

In his lab, Harris gave mountain yellow-legged frogs infected with chytrid a bath containing J. liv. The results were better than he could have imagined: 100 percent of the frogs survived, while those not treated died. Then he repeated the experiment in the wild. After a year, 39 percent of the frogs treated with J. liv were still alive, while those not treated were never found. Harris thought he’d found a way to fight the plague. However, after more testing, he found that the J. liv therapy didn’t work for all frog species. Harris believes that’s because certain species’ microbiomes might prevent J. liv from surviving on their skin.

Harris wiped the skin of frogs. He moved any bacteria to petri dishes that held nutrient-rich agar jelly. Then he cultured, or grew, the different microbes. To test their antifungal properties, he exposed the dishes to Bd. “Some bacteria did nothing, and some even helped the fungus thrive,” says Harris. But one bacterium killed the fungus. It’s called Janthinobacterium lividum, or J. liv. It produces a purple chemical called violacein. This substance was already known to have antifungal properties.

Harris tried it on mountain yellow-legged frogs infected with chytrid. In his lab, he gave them a bath containing J. liv. The results were better than he could have imagined. All of the frogs survived, but those not treated died. Then he repeated the experiment in the wild. After a year, 39 percent of the frogs treated with J. liv were still alive. Those not treated were never found. Harris thought he’d found a way to fight the infection. But he did more testing. He found that the J. liv treatment didn’t work for all frog species. Harris think he knows why. Certain species’ microbiomes might stop J. liv from surviving on their skin.

Researchers like Harris and Bletz continue to collect samples from the skin of frogs and salamanders to identify other probiotic bacteria that can fight chytrid. “We have a culture database of more than 7,000 bacteria,” Bletz says. “About 1,000 show strong antifungal properties.”

The next step is figuring out how to deploy these therapies in the wild. It’s unrealistic to bathe every amphibian around the world in a probiotic bath. “One thing we’re exploring is environmental augmentation,” Bletz says. It’s the idea of humans introducing bacteria to improve an environment. For instance, scientists could add a probiotic to a pond, where it would spread to the amphibian population to help reduce chytrid infections.

Before any of this happens, though, scientists first have to study how the introduction of probiotic bacteria would affect an ecosystem. “We would only increase bacteria that are naturally part of the environment,” explains Harris. “That would optimize the probiotics in the pond without impacting any other species.” It could take years to fully figure out this complicated puzzle, but hope for amphibians may be on the horizon. 

Researchers like Harris and Bletz are collecting more samples from the skin of frogs and salamanders. They’re finding other probiotic bacteria that can fight chytrid. “We have a culture database of more than 7,000 bacteria,” Bletz says. “About 1,000 show strong antifungal properties.”

How will they get these treatments into the wild? The next step is to figure that out. They can’t wash every amphibian around the world in a probiotic bath. “One thing we’re exploring is environmental augmentation,” Bletz says. It’s the idea of humans adding bacteria to improve an environment. For example, scientists could add a probiotic to a pond. Then it would spread to the local amphibians to help reduce chytrid infections.

But before they do this, scientists must answer a question. How would the added probiotic bacteria affect an ecosystem? “We would only increase bacteria that are naturally part of the environment,” explains Harris. “That would optimize the probiotics in the pond without impacting any other species.” It could take years to solve this difficult puzzle completely. But hope for amphibians may be on the way.