Crisis biology: Can bacteria save bats and frogs from deadly diseases?

As populations plummet, biologists race for a solution.

  • Biologist Vance Vredenburg is one of several researchers turning to microbes in hopes of saving species threatened by disease.

    Anand Varma
  • Vredenburg swabs a Sierra Nevada yellow-legged frog to test for the presence of chytrid fungus in Kings Canyon National Park, California.

    Anand Varma
  • A little brown bat with white-nose syndrome in Greeley Mine, Vermont, March 26, 2009.

  • Dish A shows P. destructans (the fungus that causes white-nose syndrome in bats) spore growth after 21 days. Spores still grew in Dish B with unactivated R. rhodochrous bacteria. In Dish C, activated bacteria completely inhibited spore growth after 21 days. (When this experiment was conducted the, the fungus was named Geomyces destructans; it has since has been changed to Pseudogymnoascus destructans.)

    Christopher Cornelison
 

In 2007, Valerie McKenzie volunteered for a large study of human body bacteria. It was the dawn of the golden age of the microbe. Researchers were just beginning to understand how bacteria and other microbes in human intestines influence everything from obesity to allergies and infections. McKenzie, a University of Colorado-Boulder biologist, was mildly curious about her "microbiome." But she was more interested in the bacteria living on the skin of frogs and toads.

Amphibian populations worldwide are plummeting, and entire species are going extinct. The West's struggling species include boreal toads and mountain yellow-legged frogs. Invasive species and habitat degradation play a major role, but amphibians are dying even in places with good habitat. Batrachochytrium dendrobatidis, an aggressive fungus commonly known as chytrid, is often to blame.

McKenzie, who was studying the role of farmland conversion and suburbanization in the decline of leopard frogs in Colorado, suspected chytrid was also a factor. When she read a paper about a strain of bacteria found on red-backed salamanders that inhibited chytrid's growth, she began to wonder: What microbes lived on the skin of her frogs and toads? And could any of them fight chytrid?

She captured boreal toads and leopard frogs and swabbed their legs, feet and bellies at research sites near Boulder and Meeker, Colo. She took the samples to Rob Knight, a fellow CU-Boulder biologist who studies the human microbiome. In exchange for DNA analyses of the bacteria on her amphibians, McKenzie volunteered for Knight's study, handing over fecal samples and forehead, armpit and hand swabs. "I was like, 'Whatever you want, can you just run some of my frog samples?' " she laughs. "That's how I got my first data set."

McKenzie and others now hope to harness the breakthroughs of the human-microbe revolution to slow two of wildlife's most prolific – and seemingly unstoppable – modern killers: chytrid, "the worst infectious disease ever recorded among vertebrates," according to the International Union for Conservation of Nature, and white-nose syndrome, a fungal disease that has killed more than 5.7 million bats in North America since 2006.

"When you see almost 99 percent of your population perished on the (cave) floor, it's incredibly alarming," says Tina Cheng, a University of California-Santa Cruz Ph.D. student who studies white-nose syndrome. "People are desperate for solutions."

Chytrid is a cruel and efficient killer. It spreads through water and skin-to-skin contact – during mating, for instance – infecting amphibians' skin, reducing their ability to absorb water and disrupting electrolyte levels enough to cause major organ failure. In weeks, it can obliterate populations.

Scientists suspect the disease is so devastating because most amphibians have never encountered this strain of the fungus before. Humans likely transported it around the globe in the pet trade, on American bullfrogs, whose legs are a delicacy, or on African clawed frogs, which doctors used for pregnancy tests until the 1970s. (The frog laid eggs when injected with a pregnant woman's urine.) Scientists first linked mass mortalities to chytrid in Australia and Central America in the late 1990s. It's since driven more than 200 species to collapse or extinction and colonized every continent except Antarctica.

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