A frightening superbug yeast that’s killing people in hospitals can also survive just fine outside of them, according to a new study out Tuesday. For the first time, researchers say they’ve discovered multidrug-resistant strains of the fungus Candida auris in a natural environment, in the remote wetlands of India. The findings indicate that these sorts of environments could be the yeast’s native home, while also providing evidence that warming temperatures due to climate change have recently made the fungus dangerous to humans, as some scientists have theorized.
C. auris was first discovered in 2009 by doctors in Japan, who isolated it from a patient’s ear infection (the first known cases date back to the mid-1990s, however). Since then, the yeast has been found in over a dozen countries, including the U.S. It can cause life-threatening infections, especially in already weakened hospital patients. But what makes the yeast especially scary is that it’s often resistant to multiple antifungal medications at once, making these infections difficult to treat and frequently fatal. The fungus is also a survivor outside of the human body, so once it’s established somewhere, it’s incredibly hard to remove it from the environment. If that wasn’t enough, C. auris can’t be identified easily through conventional tests, which can delay care and increase the risk of death.
There have only been around 1,600 cases of the yeast infection identified in the U.S. since 2009, but it’s considered one of the most serious emerging germ threats we face today. That threat has made understanding its origins and likely recent introduction to people all the more important. This new study, published in mBio on Tuesday, seems to provide the first real clues to that mystery.
Researchers in India and Canada went looking in environmental niches of India largely isolated from humans that could have been habitable for the yeast, based on its known biology and that of related species. They collected soil and water samples from the coastal wetlands of the Andaman Islands, an archipelago not far from the mainland. In two of the eight sites they searched—a salt marsh and a sandy beach—they found the fungus. The team found strains of C. auris that were susceptible and resistant to antifungals, and these strains bore a close genetic resemblance to strains collected from patients in India.
Altogether, their work on C. auris suggests that “prior to its recognition as a human pathogen, it existed as an environmental fungus,” the authors wrote.
Compared to other species of Candida, C. auris is known to thrive especially well in warmer temperatures. That’s made some researchers wonder if climate change played a part in in its emergence as a human germ. The theory argues that changes to the climate in their natural environment led the yeast to slightly adapt and become even more tolerant of warmer temperatures—the exact sort of temperatures that would make humans and other mammals a comfortable home once the yeast started regularly coming into contact with us.
The new findings seem to add more weight to that theory. Aside from showing that these fungi can and do live far from people, the team found subtle differences between the samples they found. One strain of yeast found in the more remote salt march was slower to grow under warmer temperatures than the strains found on the sandy beach and another salt march strain; this strain was also the only one found to be susceptible to common antifungals and less closely related to the strains seen in people. Meanwhile, the other strains were all resistant to antifungals and more warm-loving. The strains found on the beach, where people do visit sometimes, could have been reintroduced to the environment by humans, which could explain why they were more closely related to the strains found in patients.
It’s possible that the researchers have essentially collected snapshots of the yeast’s evolutionary journey, before and after climate change started to alter their biology and they first infected people. In an accompanying commentary written by some of the researchers who first proposed this theory—Arturo Casadevall from Johns Hopkins, Dimitrios Kontoyiannis from The University of Texas MD Anderson Cancer Center, and Vincent Robert from the Westerdijk Fungal Biodiversity Institute in the Netherlands—they agreed with those conclusions.
“This landmark discovery is crucial for understanding the epidemiology, ecology, and emergence of C. auris as a human pathogen,” they wrote.
In a statement released by the American Society For Microbiology, which publishes mBio, lead author Anuradha Chowdhary, a medical mycobiologist at the University of Delhi in India, said: “This study takes the first step in toward understanding how this pathogen survives in the wetland, but this is just one niche.”
The findings are still just one study’s worth, so they alone don’t prove that climate change introduced this latest nightmare into our lives, which the authors acknowledge. And there’s still a lot to be solved about how and from where C. auris emerged from the wild and into our hospitals, not to mention whether there’s anything that can be done to stop its spread.