A study published in Cell Reports Medicine identified the gut bacterium Bacteroides uniformis and its indole metabolites as key contributors to enhanced anti-tumor immunity in melanoma. The research marks the first demonstration that metabolites produced by specific gut bacteria can positively influence the body’s immune response to cancer.

The study pinpointed Bacteroides uniformis as a gut bacterium capable of boosting the body’s ability to fight melanoma. Researchers found that both Bacteroides uniformis and the metabolites it produces help suppress tumor growth. The bacterium converts the amino acid tryptophan into indoles, which were shown to boost anti-tumor immunity in mice.

Investigators analyzed samples from cancer patients receiving immunotherapy and discovered increased levels of enzymes used to produce indoles in those who responded well to treatment. The anti-tumor effect was dependent on the bacterium’s ability to generate indoles: only the indole-producing bacterial strain produced anti-tumor immunity in the study. When a genetically modified Bacteroides uniformis that could not convert tryptophan into indoles was introduced to germ-free mice, the anti-tumor effect disappeared, and tumors developed normally in those mice.

The research was co-led by Amanda Ramer-Tait, Maxcy Professor in Food Science and Technology and director of Nebraska’s Gnotobiotic Mouse Program, and Ze’ev Ronai, director of the Translational Research Institute at Cedars-Sinai and professor of surgery. The study included researchers from the University of Nebraska–Lincoln and collaborators at Cedars-Sinai and other institutions. Researchers used Nebraska’s Gnotobiotic Mouse Program to distinguish the effects of tryptophan degradation, a function supported by the program’s controlled experimental environment.

Ronai noted that prior research had established gut bacteria’s influence on the immune system and cancer defense. “Previous research established that gut bacteria affect the immune system and can help fight cancer,” Ronai said. He added, “Our new findings are an important step forward because they give us a specific metabolite that can be used for future therapies.”

Ramer-Tait emphasized the clinical implications of understanding microbiome variability among patients. “When I think about the role of the microbiome in cancer therapy, I see an opportunity to better understand why some patients respond well to treatments such as immune checkpoint inhibitors while others don't,” she said. She added, “Our study advances that idea by identifying a specific microbe and the metabolites it produces as one potential reason why patients respond differently to immunotherapies.”

Looking ahead, Ramer-Tait outlined potential therapeutic strategies rooted in the findings. “Our idea is that down the road, we could design microbiome- or diet-based interventions that provide beneficial microbes or the dietary substrates they need to enhance a patient's response to immunotherapy,” she said. She also the broader applicability of the mechanism: “What's especially exciting is that this approach has the potential to extend beyond melanoma because indoles play an important role in improving immune responses to other types of cancers as well.”

The study’s experimental design relied on germ-free mouse models and genetic modification to isolate the role of indole production. Tumors developed normally in mice that received the genetically modified, non-indole-producing strain, confirming that indole metabolites—not the bacterium alone—were responsible for the anti-tumor effect. This causal link was further supported by human data showing elevated indole-producing enzyme levels in immunotherapy responders.

The findings offer a mechanistic explanation for variability in immunotherapy outcomes and identify a precise microbial and metabolic target for future cancer treatments. By linking Bacteroides uniformis, tryptophan metabolism, and indole production to anti-tumor immunity, the study provides a foundation for microbiome-modulating therapies that could improve responses to existing immunotherapies.

The research was supported by Nebraska’s Gnotobiotic Mouse Program and funded by the National Institutes of Health and the University of Nebraska Medical Center’s Fred and Pamela Buffett Cancer Center. Study authors included Kristin Beede, research lab manager for Nebraska’s Gnotobiotic Mouse Program. The paper is available in Cell Reports Medicine under DOI 10.1016/j.xcrm.2026.102921.