Fungal DNA is present in various types of cancer, according to two studies published yesterday (September 29) in Cell. The findings add support to a hypothesized link between fungi and certain cancers, although researchers emphasize that there isn’t yet evidence for a causal connection.

The studies provide “pretty compelling evidence there may be rare fungi within tumors,” Stanford University’s Ami Bhatt, who was not involved in either study, tells STAT. She adds that the work also raises various questions about the detected fungi: “Are they alive or not? And assuming they really are there, then why are they there? And how did they get there?”

Both studies examined tissues from multiple types of human cancers throughout the body. One group, led by researchers at the Weizmann Institute of Science in Israel and the University of California, San Diego (UCSD), reports detecting fungal DNA or cells—typically at low abundance—in 35 different cancer types, with fungal species composition differing among them.

See “Could Cancer’s Microbiome Help Diagnose and Treat the Disease?

The discovery of fungi was “surprising because we don’t know how fungi could get into tumors,” study coauthor Rob Knight, a microbiome researcher at UCSD and a cofounder of Micronoma, which develops methods of diagnosing and treating cancer using microbial biomarkers, says in a press statement. “But it is also expected because it fits the pattern of healthy microbiomes throughout the body, including the gut, mouth and skin, where bacteria and fungi interact as part of a complex community.”

The other team, led by researchers at Duke University and Cornell University, also found evidence of fungi in multiple cancer types, and highlighted a particular association between Candida species and gastrointestinal cancers. Across various sites, “several Candida species were enriched in tumor samples and tumor-associated Candida DNA was predictive of decreased survival,” the authors write in their paper.

It remains unclear whether the fungus is playing any role in cancer pathology, and there are possible alternative explanations for the connection. For example, fungi might simply be taking advantage of immune system suppression in cancerous tissues, Bhatt tells STAT. “Or maybe there are immune cells that ate fungi and carried sequences to a tumor site. . . . Or maybe since you have a trillion microbes in and on you, it’s just not surprising that every now and then one makes its way into the body.”

See “Bacteria in Tumors Promote Metastasis in Mice

Charis Eng, a cancer geneticist at Cleveland Clinic in Ohio who was not involved in the research, tells Nature that researchers will have to carry out in vitro and in vivo experiments to examine whether the fungi encourage healthy cells to become cancerous before they can establish a causal connection or target that connection therapeutically.

In the meantime, both papers propose that the presence of fungi in tumors might offer a new diagnostic or prognostic biomarker for cancer pathology. And the Weizmann Institute’s Ravid Straussman, a coauthor on one of the studies who is also on Micronoma’s scientific advisory board and consults for various cancer- and microbiome-related biotech companies, tells STAT that he wants to see more cancer scientists consider the microbiome in their research.

“As a field, we need to evaluate everything we know about cancer,” Straussman says. “Look at everything through the lens of the microbiome—the bacteria, the fungi, the tumors, even viruses. There are all these creatures in the tumor, and they must have some effect.”