The most evident link yet between fungi and cancer is provided by studies that examined thousands of tumor samples; however, additional research is required.
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| Some tumor samples have been found to contain the fungus Candida, which can be seen growing in this laboratory. Credit: Nicholas Armor/dpa/Alamy |
There has been mounting evidence for many years that bacteria are associated with cancer and sometimes even play a significant role in its progression. Researchers have now discovered a connection similar to this one with another kind of microorganism: fungi.
According to two studies that were published on September 29 in the journal Cell, the different species of microscopic or single-celled fungus that are found in tumors of various types of cancer may one day be useful for diagnosing cancer or predicting its course. Ami Bhatt, a microbiome specialist at Stanford University in California, says, "It's fascinating to look at fungi in the context of cancer. "However, she cautions that the studies do not demonstrate whether or not the fungi are directly responsible for the progression of cancer; rather, they only suggest that there is an association between certain types of fungal species and cancers.
Fungal microorganisms, like bacteria, are an important part of the human microbiome, which is a delicate balance of microbes that live inside the body. Lian Narunsky Haziza, a cancer biologist at the Weizmann Institute of Science in Rehovot, Israel, and her colleagues cataloged fungal populations in more than 17,000 tissue and blood samples from 35 types of cancer to learn how this composition might change in cancer patients.
Fungi, including several types of yeast, were found in all of the cancers studied, as was to be expected. However, depending on the cancer, some species were linked to different outcomes. The researchers discovered, for instance, that the presence of the fungus Malassezia globosa, which has previously been linked to pancreatic cancer, was associated with significantly lower survival rates in breast cancer. In contrast to typical environments, in which fungi and bacteria compete for shared resources, Narunsky Haziza and her colleagues discovered that the majority of fungi had certain bacterial species with which they tended to coexist in the tumors by also characterizing the bacteria.
Immunologist Iliyan Iliev and his colleagues at Weill Cornell Medicine in New York City looked at gastrointestinal, lung, and breast tumors in another study. They found that they typically contained fungi from Candida, Blastomyces, and Malassezia, respectively. The researchers discovered that gastrointestinal tumor cells with higher levels of Candida had lower survival rates, a higher rate of metastasis, and more gene activity that encourages inflammation.
According to New York University microbiologist Deepak Saxena, who has investigated the connection between fungi and cancer, characterizing fungal cells in a tumor is like finding a needle in a haystack. According to him, the ratio of fungal cells to tumor cells can vary from sample to sample.
Iliev says that sample contamination is a serious concern because many of the fungi in question are widespread. Because of this, the researchers had to be very careful not to include any false matches for fungal DNA or possible signs of contamination in their findings. Iliev and his colleagues, for instance, discovered tumor tissues from all over the body where a DNA fragment was mistakenly identified as portobello mushroom (Agaricus bisporus), a common edible fungus.
According to Bhatt, the research teams did not collect the samples with the intention of minimizing fungal contamination because the majority of the tissue and blood samples came from databases. She would like to see the results replicated using sterile samples, despite the fact that the researchers developed methods to remove any potential contaminants from the sequencing data.
Effects of fungi Despite the fact that this study establishes the most definitive link yet between fungi and cancer, Saxena asserts that additional research is required to determine whether fungi can contribute to the progression of cancer by, for instance, causing inflammation, or whether advanced tumors create an environment that is habitable and encourages fungal cells to take root.
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