Last year, the controversy heightened when experts questioned a high-profile study — a 2020 analysis claiming that the tumors of 33 different cancers had their own unique microbiomes — on whether the “signature” of these bacterial compositions could help diagnose cancer.
The incident renewed the spotlight on “tumor microbiomes” because of the bold claims of the original paper and the strongly worded refutations of those claims. The broader field has focused primarily on ways the body’s microbiome interacts with cancers and cancer treatment.
This controversy has highlighted the challenges of making headway in a field where researchers may not even have the tools yet to puzzle-out the wide-ranging implications the microbiome holds for cancer diagnosis and treatment.
But it is also part of a provocative question within that larger field: whether tumors in the body, far from the natural microbiome in the gut, have their own thriving communities of bacteria, viruses, and fungi. And, if they do, how do those tumor microbiomes affect the development and progression of the cancer and the effectiveness of cancer therapies?
Cancer Controversy
The evidence is undeniable that some microbes can directly cause certain cancers and that the human gut microbiome can influence the effectiveness of certain therapies. Beyond that established science, however, the research has raised as many questions as answers about what we do and don’t know about microbiota and cancer.
The only confirmed microbiomes are on the skin and in the gut, mouth, and vagina, which are all areas with an easy direct route for bacteria to enter and grow in or on the body. A series of papers in recent years have suggested that other internal organs, and tumors within them, may have their own microbiomes.
“Whether microbes exist in tumors of internal organs beyond body surfaces exposed to the environment is a different matter,” said Ivan Vujkovic-Cvijin, PhD, an assistant professor of biomedical sciences and gastroenterology at Cedars-Sinai Medical Center in Los Angeles, whose lab studies how human gut microbes affect inflammatory diseases. “We’ve only recently had the tools to study that question on a molecular level, and the reported results have been conflicting.”
For example, research allegedly identified microbiota in the human placenta nearly one decade ago. But subsequent research contradicted those claims and showed that the source of the “placental microbiome” was actually contamination. Subsequent similar studies for other parts of the body faced the same scrutiny and, often, eventual debunking.
“Most likely, our immune system has undergone selective pressure to eliminate everything that crosses the gut barrier because there’s not much benefit to the body to have bacteria run amok in our internal organs,” Dr. Vujkovic-Cvijin said. “That can only disrupt the functioning of our tissues, to have an external organism living inside them.”
The controversy that erupted last summer, surrounding research from the lab of Rob Knight, PhD, at the University of California, San Diego, centered on a slightly different but related question: Could tumors harbor their own microbiomes?
This news organization spoke with two of the authors who published a paper contesting Dr. Knight’s findings: Steven Salzberg, PhD, a professor of biomedical engineering at John Hopkins Medicine, Baltimore, Maryland, and Abraham Gihawi, PhD, a research fellow at Norwich Medical School at the University of East Anglia in the United Kingdom.
Dr. Salzberg described two major problems with Dr. Knight’s study.
“What they found were false positives because of contamination in the database and flaws in their methods,” Dr. Salzberg said. “I can’t prove there’s no cancer microbiome, but I can say the cancer microbiomes that they reported don’t exist because the species they were finding aren’t there.”
Dr. Knight disagrees with Dr. Salzberg’s findings, noting that Dr. Salzberg and his co-authors did not examine the publicly available databases used in his study. In a written response, he said that his team’s examination of the database revealed that less than 1% of the microbial genomes overlapped with human ones and that removing them did not change their findings.
Dr. Knight also noted that his team could still “distinguish cancer types by their microbiome” even after running their analysis without the technique that Dr. Salzberg found fault with.
Dr. Salzberg said that the database linked above is not the one Dr. Knight’s study used, however. “The primary database in their study was never made public (it’s too large, they said), and it has/had about 69,000 genomes,” Dr. Salzberg said by email. “But even if we did, this is irrelevant. He’s trying to distract from the primary errors in their study,” which Dr. Salzberg said Dr. Knight’s team has not addressed.
The critiques Dr. Salzberg raised have been leveled at other studies investigating microbiomes specifically within tumors and independent of the body’s microbiome.
For example, a 2019 study in Nature described a fungal microbiome in pancreatic cancer that a Nature paper 4 years later directly contradicted, citing flaws that invalidated the original findings. A different 2019 study in Cell examined pancreatic tumor microbiota and patient outcomes, but it’s unclear whether the microorganisms moved from the gut to the pancreas or “constitute a durably colonized community that lives inside the tumor,” which remains a matter of debate, Dr. Vujkovic-Cvijin said.
A 2020 study in Science suggested diverse microbial communities in seven tumor types, but those findings were similarly called into question. That study stated that “bacteria were first detected in human tumors more than 100 years ago” and that “bacteria are well-known residents in human tumors,” but Dr. Salzberg considers those statements misleading.
It’s true that bacteria and viruses have been detected in tumors because “there’s very good evidence that an acute infection caused by a very small number of viruses and bacteria can cause a tumor,” Dr. Salzberg said. Human papillomavirus, for example, can cause six different types of cancer. Inflammation and ulcers caused by Helicobacter pylori may progress to stomach cancer, and Fusobacterium nucleatum and Enterococcus faecalis have been shown to contribute to colorectal cancer. Those examples differ from a microbiome; this “a community of bacteria and possibly other microscopic bugs, like fungi, that are happily living in the tumor” the same way microbes reside in our guts, he said.
Dr. Knight said that many bacteria his team identified “have been confirmed independently in subsequent work.” He acknowledged, however, that more research is needed.
Several of the contested studies above were among a lengthy list that Dr. Knight provided, noting that most of the disagreements “have two sides to them, and critiques from one particular group does not immediately invalidate a reported finding.”
Yet, many of the papers Dr. Knight listed are precisely the types that skeptics like Dr. Salzberg believe are too flawed to draw reliable conclusions.
“I think many agree that microbes may exist within tumors that are exposed to the environment, like tumors of the skin, gut, and mouth,” Dr. Vujkovic-Cvijin said. It’s less clear, however, whether tumors further from the body’s microbiome harbor any microbes or where they came from if they do. Microbial signals in organs elsewhere in the body become faint quickly, he said.