Scientists have discovered a type of fungus that has never been seen in humans before. They say warmer temperatures may be making it evolve for the worse. The fungus, called Rhodosporidiobolus fluvialis, was found in clinical samples from two unrelated hospital patients. In experiments, the researchers found that the yeast was resistant to several first-line antifungal drugs at higher temperatures – around human body temperature. This temperature also led to the emergence of “hypervirulent mutants” that could cause more severe illness in lab mice.
The results “support the idea that global warming may facilitate the evolution of new fungal pathogens,” the researchers who made the discovery wrote in a report published June 19 in the journal Nature Microbiology.
Scientists made the discovery after studying fungal samples taken from patients at 96 hospitals across China between 2009 and 2019. A total of 27,100 fungal strains were collected and analyzed; of these, only R. fluvialis had never been seen in humans before.
R. fluvialis was found in the blood of two unrelated patients who, in addition to being infected with the yeast, had serious underlying medical conditions. One patient was a 61-year-old who died in an intensive care unit (ICU) in Nanjing in 2013, and the other was an 85-year-old who died in 2016 after being treated in an ICU in Tianjin. The report does not indicate whether the fungal infection directly contributed to the deaths of these patients or whether they were simply infected at the time.
As part of the treatment, patients were given common antifungal drugs, including fluconazole and caspofungin. Laboratory tests conducted by the team later showed that R. fluvialis was resistant to both of these drugs.
The emergence of new invasive fungal pathogens, like the one found in the new study, poses a serious public health threat. TopMicrobialStock
Invasive fungal infections, which attack tissue deep within the body, mostly affect people whose immune systems are weakened, such as by HIV infection or immunosuppressant medications. Notably, the 61-year-old man infected with R. fluvialis was immunosuppressed, and the 85-year-old man had diabetes, which can impair immune function.
But rising global temperatures have forced fungi to adapt and expand their geographic ranges, making some more likely to come into contact with humans. This has led to the emergence of new pathogens, including drug-resistant Candida auris, which has been identified in more than 40 countries since its discovery in 2009. Meanwhile, the development of new antifungal drugs has largely stalled, leaving few options for combating resistant infections.
In the new study, the researchers infected immunodeficient mice with R. fluvialis and found that some of the fungal cells quickly evolved to grow more aggressively. The team then looked at the fungus in lab dishes that were kept at human body temperature — 98.6 degrees Fahrenheit (37 degrees Celsius). At that temperature, the yeast mutated 21 times faster than at room temperature, about 77 F (25 C).
Heat also increased the likelihood that R. fluvialis would develop drug resistance. When exposed to the antifungal drug amphotericin B, the yeast developed resistance more quickly at body temperature than at room temperature.
The scientists write in their paper that if yeasts like R. fluvialis are more likely to become virulent and drug-resistant at high temperatures, global warming could potentially facilitate the emergence of dangerous new fungal pathogens.
But when it comes to R. fluvialis in particular, some scientists are wary of jumping to alarming conclusions. Matthew Fisher, a professor of fungal epidemiology at Imperial College London who was not involved in the study, said the yeast should not yet be seen as a serious, new threat.
“My first feeling here is that there are unexplored environments in China where this yeast is present, and that these two patients were unlucky enough to be exposed,” he told Science magazine.
