Gut Microbiome Shapes Cancer Immunotherapy Outcomes Across New Studies

The specific mix of bacteria living in a person's gut can predict the chances that melanoma will recur after surgery and immunotherapy. Published online April 17 in the journal Cell, the study in 674 patients enrolled in a global clinical trial revealed that differences in the amount of key bacterial groups (taxa) in the gut predicted cancer recurrence with up to 94% accuracy when patients were analyzed in a new way. The taxa associated most with changes in recurrence risk were Eubacterium, Ruminococcus, Firmicutes, and Clostridium.

The study found that bacterial markers that most accurately signal recurrence risk varied depending on where a patient lived. By first matching patients based on the overall similarity of their gut microbiomes regardless of where they lived, the research team found bacterial “fingerprints” that predict cancer recurrence accurately in each region. Levels of these markers predicted cancer recurrence ranging from 83 to 94 percent accuracy, depending on the geographical region. The gut microbiome remained remarkably stable in patients during their year-long course of immunotherapy, meaning a single pre-treatment microbiome test could provide a reliable forecast of a patient's risk.

Another study, published in Nature, examined the relationship between diet, the gut microbiome, and immunotherapy response. The study, “Diet–microbiome synergy underlies obesity-associated immunotherapy efficacy,” demonstrated that the interaction between diet and the gut microbiome plays a central role in shaping antitumor immunity. Using 12 distinct dietary models representing a broad spectrum of human eating patterns, the investigators found that body weight and metabolic dysfunction were poor predictors of immunotherapy response. Dietary composition—not obesity itself—was the dominant factor influencing immune checkpoint inhibitor sensitivity.

A study from a team including University of Nebraska–Lincoln researchers, published in Cell Reports Medicine, was the first to show that metabolites produced by certain bacteria in the gut can positively impact the body's immune response to cancer. The team showed that Bacteroides uniformis and the metabolites it produces can help suppress tumor growth in melanoma. The bacterium converts the amino acid tryptophan into indoles, which boosted anti-tumor immunity in mice. In cancer patients receiving immunotherapy, investigators found increased levels of enzymes used to produce indoles in those responding well to the therapy. The findings suggest that manipulating the gut microbiome—or providing beneficial bacterial metabolites directly—could someday help improve responses to cancer immunotherapy, potentially extending beyond melanoma.

A review in Cancer Biology & Medicine from researchers at The Chinese University of Hong Kong explored how the gut microbiome affects immunotherapy responses in hepatocellular carcinoma (HCC). The review reported that in HCC, dysbiosis takes hold: harmful species like Klebsiella pneumoniae and Catenibacterium mitsuokai expand, producing toxic metabolites like deoxycholic acid and quinolinic acid that drive chronic inflammation and directly promote tumor growth. Specific microbial signatures linked to immunotherapy resistance include enrichment of Phocaeicola vulgatus, which impairs CD8+ T-cell cytotoxicity by disrupting tryptophan metabolism and depleting indole-3-acetic acid. The review highlighted potential interventions such as Bifidobacterium supplementation, dietary fiber, and fecal microbiota transplantation, and noted an ongoing trial, FAB-HCC, combining FMT with atezolizumab and bevacizumab.

A pilot study presented at the 2026 Oncology Nursing Society Congress examined gut microbiome patterns and symptoms in 15 melanoma patients receiving immune checkpoint inhibitors. Fatigue increased during early ICI therapy, whereas pain, sleep disturbance, depression, and cognitive symptoms remained comparatively stable. Specific bacteria, including Oscillibacter and Tannerellaceae, were associated with lower levels of fatigue or pain, while Lactobacillus and Limosilactobacillus were associated with higher symptom levels. Researchers also observed a decrease in Blautia during treatment.

In a clinical trial led by Oregon State University scientists, published in Veterinary Oncology, canine cancer patients receiving a new form of immunotherapy lived longer or shorter depending on the composition of their gut microbiome. The study involved 51 dogs diagnosed with a range of malignancies. Scientists found 11 specific types of bacteria linked to how long the dogs lived—some associated with longer survival, others with shorter survival. The findings suggest a microbiome swab could help predict how well a dog might respond to cancer treatment.

The Society for Immunotherapy of Cancer (SITC) will host a session, “Microbiome and Metabolism,” at SITC26 on Nov. 7, examining the microbiome's growing role in cancer development, tumor immunity, and response to therapy.