Do fungi that hide inside cancers accelerate their growth?

the fungus candidate – shown here growing under laboratory conditions – has been found in some tumor samples.Credit: Nicolas Armer/dpa/Alamy

For years, evidence has been mounting that bacteria are linked to cancer, and sometimes even play a crucial role in its progression. Now, researchers have found a similar connection with another type of microorganism: fungi.

According to two studies published in cell on September 291,2. “It’s fascinating to look at fungi in the context of cancer,” says Ami Bhatt, a microbiome specialist at Stanford University in California. But he cautions that studies only suggest an association between fungal species and certain cancers; they do not show whether or not fungi are directly responsible for cancer progression.

Internal microorganisms

Like bacteria, fungal microorganisms form a crucial part of the human microbiome: a delicate balance of microbes that live inside the body. To understand how this composition may be altered in people with cancer, Lian Narunsky Haziza, a cancer biologist at the Weizmann Institute of Science in Rehovot, Israel, and colleagues cataloged fungal populations in more than 17,000 tissue samples and blood representing 35 types of cancer.1.

As expected, fungi, including several types of yeast, were present in all types of cancer included in the study, but some species were linked to different outcomes, depending on the cancer. For example, the presence of Malassezia globosa, a fungus that has previously been associated with pancreatic cancer, was linked to significantly reduced survival rates in breast cancer, the researchers found. By also characterizing the bacteria in the tumors, Narunsky Haziza and colleagues found that most types of fungi had certain bacterial species that they tended to coexist with, meaning that the tumor could favor the growth of both fungi and of bacteria, unlike typical environments, in which fungi and bacteria compete for shared resources.

In another study2immunologist Iliyan Iliev of Weill Cornell Medicine in New York City and colleagues looked at gastrointestinal, lung and breast tumors and found that they tended to contain candidate, Blastomyces i Malassezia fungi, respectively. Higher levels of candidate The researchers found that gastrointestinal tumor cells were linked to more gene activity that promotes inflammation, a higher rate of metastasis and lower survival rates.

Characterizing fungal cells in a tumor is like finding a needle in a haystack, says Deepak Saxena, a New York University microbiologist who has researched the fungus-cancer connection. Depending on the sample, there is usually only about one fungal cell for every 10,000 tumor cells, he says.

Also, many of the fungal species in question are widespread, making sample contamination a serious concern, Iliev says. This meant that the researchers had to be very careful to filter out any potential evidence of contamination or false matches for fungal DNA from their results. For example, Iliev and colleagues found a DNA fragment misidentified as a portobello mushroom (Agaricus bisporus), a common edible fungus, in tumor tissues throughout the body.

The two research teams obtained most of their tissue and blood samples from databases, so the samples were not collected with the goal of minimizing fungal contamination, Bhatt says. Although the researchers developed methods to filter out any potential contaminants from the sequencing data, they would like to see the results replicated using samples taken in a sterile environment.

Effects of fungi

While this research provides the clearest link yet between cancer and fungi, Saxena says more work is needed to understand whether fungi can contribute to cancer progression by causing inflammation, for example, or whether advanced tumors create a habitable environment that favors the fungal cells to attach. .

Answering these questions will require researchers to investigate one type of cancer at a time and use lab-grown cells and animal models to test whether fungi encourage healthy cells to become cancerous, says Charis Eng, a Cleveland cancer geneticist. Clinic in Ohio. Once researchers more fully understand the role of fungi in cancer, they may be able to develop therapeutics or probiotics that control fungal populations, which could help stop cancer progression, says Eng.

It will also be important to piece together how bacteria, viruses and fungi interact and contribute to cancer, says Nadim Ajami, a microbiome specialist at MD Anderson Cancer Center in Houston, Texas. “We know that these ecologies coexist,” he says. “When we think only of bacteria or fungi, we tend to forget that they live in the same environment.”

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