The human body is teeming with bacteria, and a new study finds the same is true of many cancers – raising questions about what role microbes might play in the diseases.
Researchers have already known that tumours in certain areas of the body – like the gut – harbour bacteria of their own. But the new research reveals that a range of cancers, including those of the breast, lungs, bones and brain, have their own bacterial communities within tumour cells.
And the makeup of those communities seems to be unique to each cancer type, the researchers report in the 29 May issue of Science.
Exactly what it all means is unclear at this point. But the study also found that the same bacteria within tumour cells were present in patients' immune system cells. Among other things, that brings up the possibility that tumour bacteria could have an effect on patients' responses to certain cancer therapies.
Role of gut bacteria
"The fact that bacteria are frequently present in human tumours raises many intriguing questions regarding their local effects on tumour cells, as well as on tumour immunity and response to immunotherapy," said senior researcher Dr Ravid Straussman, of the Weizmann Institute of Science in Rehovot, Israel.
Past research has already turned up hints that bacteria may influence cancer patients' response to treatment. A 2017 study of patients with advanced melanoma suggested that people's own gut bacteria might play a role: Patients who responded well to immunotherapy drugs generally had a gut "microbiome" that was distinct from that of patients who did not respond.
The new findings point to a similar connection between tumour bacteria and treatment response: Among melanoma patients, there was an association between the amount of certain bacteria in their tumours and the likelihood of responding to immunotherapy.
It is too soon, though, to draw any conclusions, according to Peter Turnbaugh, an associate professor of microbiology and immunology at the University of California, San Francisco. He co-authored an editorial published with the findings.
"The current study reports an association with immunotherapy response," Turnbaugh said. "But the causal role of bacteria within tumours, or in other body sites, remains unclear – as do the potential mechanisms that might be at play."
The results are based on more than 1 500 tumour samples, plus samples of nearby tissue, from patients at medical centres in four countries. Straussman and his colleagues analysed the samples for bacterial proteins and genetic material, taking measures to ensure they would not simply detect microbes that got there by contamination.
Bacteria may 'translocate'
It turned out that all seven cancer types they studied harboured bacterial communities within their cells. And the composition of those communities varied by cancer type. Breast cancers, for example, had the most diverse collection of bacteria.
One of the key questions, Turnbaugh said, is where are those tumour bacteria coming from?
One possibility, he speculated, is that bacteria are able to "translocate" out of a person's gut or other tissue and find their way into tumour cells. Other recent research has suggested bacteria can travel from the gut to tumours, Turnbaugh said.
Why would different cancer types have distinctive bacterial communities? The "microenvironment" within different body tissues and cancer types is known to vary, Turnbaugh explained. So it's possible those conditions favour the growth of certain bacteria over others.
However, that's not to say that all patients with a particular cancer have exactly the same bacterial makeup in their tumours. The study also found differences among individuals with the same cancer. In melanoma patients, certain differences in tumour bacterial makeup were associated with treatment response.
Straussman said it all raises many "intriguing" questions about how bacteria might interact with a patient's cancer and own immune cells. Some bacteria, he noted, might have "anti-tumour" effects, while others could be "pro-tumour".
Changing the gut's bacterial environment
If tumour bacteria do influence treatment outcomes – an area Straussman said his team is actively studying – that would lead to an obvious next step: figuring out what, if anything, can be done about it.
Straussman said ongoing research might lead to new ways of "manipulating" tumour bacteria, to improve cancer treatment.
Right now, it's not clear how bacteria within tumour cells could be targeted. But, Turnbaugh said, if it turns out that gut bacteria do migrate to tumours, it might be possible to alter tumour bacteria by changing the gut's bacterial environment.
In one instance of how such research is bearing fruit, a study presented this week at the annual meeting of the American Society of Clinical Oncologists found that bacteria in or around colon tumours may be tied to patient survival.
Researchers at Ohio State University Comprehensive Cancer Center found that "the presence of bacteria was mostly, but not always, associated with worse overall survival". They believe that testing for bacterial types might help guide treatment, as might the use of antibiotics against certain species.
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