Scientists Are Finding Fungi in Cancerous Tumors

Experts aren’t sure if the fungi affect the disease’s progression

A stained micrograph revealing the presence of the fungus Candida albicans.
The fungus Candida albicans in a patient with pulmonary candidiasis. Humans are home to trillions of harmless cells of bacteria, fungi and other microbes that play an important role in our health. Smith Collection/Gado/Getty Images

Humans live in harmony with our microbiome—a collection of microorganisms in and on our bodies. Each person could host roughly 39 trillion of these tiny organisms, such as bacteria and fungi. Scientists have long understood that the microbiome is vital to our health.

But now, two new studies show that fungi can also be found in a rather unexpected place: cancerous tumors.

The studies, published last week, provide “pretty compelling evidence there may be rare fungi” within these masses of tissue, Ami Bhatt, a microbiome specialist at Stanford University who wasn’t involved with either study, tells Stat NewsAngus Chen. But the work raises questions, such as, “Are they alive or not? And assuming they really are there, then why are they there? And how did they get there?” she says.

While the findings show an association between types of fungi and specific cancers, they don’t show whether fungi play a role in how cancer affects people, Bhatt tells Nature News Max Kozlov.

In 2020, several research teams showed that cancerous tumors contain bacteria, according to the New York TimesCarl Zimmer. Ravid Straussman, a cancer biologist at the Weizmann Institute of Science in Israel, and colleagues found bacteria in tumors from all seven types of cancer they studied, according to the Times. A separate group of researchers from the University of California, San Diego, demonstrated the same year that a small percentage of DNA from 32 types of cancer contained bacterial DNA, per the Times.

In one of the new papers, published in the journal Cell, these two teams of researchers collaborated to search for fungi in tumors. They looked at both tumor samples and the Cancer Genome Atlas database, according to Stat News. They found fungi in all 35 types of cancer they studied, Nature News reports.

The other paper, also published in Cell on the same day, found fungi in tumors from seven parts of the body, per the Times.

Straussman estimated that some tumors may have one fungal cell for every 1,000 to 10,000 cancer cells. He tells Stat News that on the higher end, “it could still be one million fungal cells in a tumor, which could have a big effect on cancer biology.”

“I was not expecting this amount of fungus in cancer,” Deepak Saxena, a microbial ecologist at New York University who did not contribute to either paper, tells the Times. “This will change the way we think about it.”

Though scientists identified more fungi than expected, it’s still a trace amount. “Finding the fungi is like looking for a needle in a haystack,” Bhatt tells Stat News. “These are very rare sequences, not tumors that are teeming with fungi.”

Bhatt tells Nature News that the researchers took most of their samples from databases that didn’t aim to minimize fungal contamination during collection. So, she’d like to see if other studies can get the same results with samples taken in a sterile environment.

Still, questions remain. Scientists don’t know what role the fungi play in cancer cells and whether they affect how the disease progresses in humans. The new studies both found that people with certain species of fungi in their tumors had worse outcomes from cancer. Some microbes could possibly help tumors grow or spread, or hide them from our immune systems, per the Times.

Saxena tells Nature News that more studies will need to be conducted to determine whether or not the fungi aid cancer's progression.

“It’s hard to tell if it’s just a correlation or if the fungi really contributed to the tumor,” Straussman says to Stat News.

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