The bacteria that live in our gut may do a lot more than help our digestion and boost our ability to fight off viruses.

They may, in fact, be lifesavers.

Two new studies in mice suggest that microorganisms living in the gut may help the immune system fight tumors.

They may also affect how well immunotherapy treatments work against certain types of cancer.

Read More: The Truth About Bacteria in the Gut »

Mice with Melanoma

In a study published today in the journal Science, researchers from the University of Chicago added a certain type of bacteria to the guts of mice with melanoma tumors.

This boosted the ability of the animal’s immune system to attack cancer cells in the tumors.

The increased immune response occurred within two weeks after feeding the mice the bacteria, known as Bifidobacterium. These antitumor effects were still present six weeks later.

Although previous research has found that gut microorganisms can influence the immune system, researchers were still somewhat surprised.

“Our results clearly demonstrate a significant, although unexpected, role for specific gut bacteria in enhancing the immune system's response to melanoma and possibly many other tumor types,” said study author Dr. Thomas Gajewski, Ph.D., a professor of medicine and pathology at the University of Chicago, in a press release.

Researchers connected the immune-boosting bacteria from the guts of mice with a naturally strong immune response to tumors implanted under the skin.

In people, melanoma is treated with a type of immunotherapy known as monoclonal antibodies that stimulates the activity of the immune system.

In this study, when the researchers gave the mice both the immunotherapy drug and the bacteria, the tumor growth slowed even more.

Read More: Immune Systems Now a Major Focus of Cancer Treatment Research »

Gut Bacteria Enhance Immunotherapy

In the other study, published in the same issue of Science, researchers from France found that certain types of gut bacteria could also affect how well immunotherapy worked in mice with melanoma.

The researchers used mice that had either been raised in a germ-free environment or treated with broad-spectrum antibiotics. This allowed the researchers to test how well the drug — another type of monoclonal antibody — worked in mice with depleted gut bacteria.

The answer is, not very well. It was only after these mice had been given a dose of another type of bacteria, called Bacteroides, that the drug regained its antitumor power.

In addition, the researchers found that giving the mice the monoclonal antibody changed which gut bacteria were present. In particular, bacteria that boost the drug’s effects were decreased.

This interaction between the microbiome and the immune system could explain why some immunotherapy treatments don’t work well for some people with cancer.

Read More: Extra Oxygen and Immunotherapy Slows Cancerous Tumor Growth »

New Avenues for Cancer Research

The use of bacteria to treat cancer isn’t a new idea. As far back as the late 19th century, scientists were attempting to harness microorganisms to stimulate the body’s immune response against tumors.

The new studies, though, show that bacteria that naturally live in our bodies and in our intestines — known as commensal bacteria — play some role in our immune system’s fight against cancer.

“What is new with these studies is the idea that commensal bacteria can actually influence immune responses against tumors that also favorably impacts therapeutic activity of [certain] immunotherapeutic interventions,” Beth A. McCormick, Ph.D., a professor of microbiology and physiological systems at the University of Massachusetts Medical School, told Healthline.

Both studies were done only in mice, so it’s difficult to say what this means for people with cancer. More research, including clinical trials, will be needed.

In addition, neither study showed that taking probiotics — “good” bacteria supplements — can prevent cancer. Only that certain bacteria in the gut can boost the immune response of mice to tumors.

“While obviously potentially very exciting and encouraging clinically, there are many steps yet to be taken,” said McCormick, “and I would be very cautious in using the term ‘anticancer cancer probiotics.’”