Colorectal cancer treatment may be influenced by an unexpected partner—or adversary—living inside the intestine. A review published in the Journal of Translational Medicine argues that the gut microbiota can help determine whether radiotherapy destroys colorectal tumors efficiently, triggers powerful antitumor immune responses, or instead causes severe intestinal injury and treatment resistance. The analysis brings together emerging evidence that radiation and resident microbes engage in a two-way biological exchange: radiation alters the intestinal ecosystem, while bacteria and their chemical products can reshape tumor radiosensitivity, immune activity, inflammation and the integrity of the gut barrier. The findings raise the prospect of tailoring cancer treatment according to a patient’s microbial profile, although the authors stress that microbiome-based therapies are not yet ready for routine clinical use.
Radiotherapy works primarily by damaging the DNA of cancer cells. Ionizing radiation can break one or both strands of the DNA molecule, generate chemically reactive oxygen species and interfere with the repair systems that tumors need to survive. Yet its effects extend beyond the directly irradiated cancer cell. Radiation can cause immunogenic cell death, a form of cellular destruction that releases tumor-associated antigens and danger signals, potentially alerting the immune system to attack malignant cells elsewhere. At the same time, radiation delivered to the pelvis inevitably exposes portions of the normal intestine, where it can injure epithelial cells, disrupt the mucus layer and provoke radiation enteritis. The review describes the microbiota as a key regulator of both sides of this equation, influencing how much damage reaches the tumor and how well healthy tissue recovers.
The intestine contains a dense and chemically active community of bacteria, archaea, fungi and other microorganisms. These organisms convert dietary components and host-derived molecules into metabolites that can act on epithelial cells, immune cells and tumor tissue. Among the most important are short-chain fatty acids, or SCFAs, including acetate, propionate and butyrate, which are produced when bacteria ferment dietary fiber. SCFAs can serve as energy sources for intestinal epithelial cells, alter gene expression through mechanisms such as histone deacetylase inhibition and influence the differentiation and activity of immune cells. In the context of radiotherapy, the review indicates that these metabolites may strengthen treatment responses by increasing tumor cell stress, supporting antitumor immunity and helping maintain the intestinal barrier. Their effects, however, depend on concentration, location and the broader microbial community in which they are produced.
One proposed route by which bacterial metabolites improve radiotherapy is by intensifying DNA damage inside cancer cells. The review highlights methylglyoxal, a reactive metabolite that can interfere with cellular proteins and promote oxidative and genotoxic stress. If a tumor cell is already struggling with defective DNA repair, additional chemical injury may push it beyond the threshold required for survival during radiation treatment. Microbial products may also influence signaling pathways that control apoptosis, cell-cycle arrest and the repair of radiation-induced DNA lesions. These mechanisms offer a biological explanation for why tumors exposed to particular microbial communities could be more radiosensitive than genetically similar tumors exposed to a different intestinal environment. The evidence is still developing, and much of it comes from experimental models rather than large patient trials, but it points to metabolism as an important connection between microbes and radiation response.
The immune system provides another link. When radiotherapy kills tumor cells in an immunogenic manner, fragments of cancer-cell DNA, proteins and other intracellular material can stimulate antigen-presenting cells. These cells then activate T lymphocytes, which may recognize and attack remaining tumor cells. Gut microbes can influence this process through microbial molecules and metabolites that interact with pattern-recognition receptors, including Toll-like receptors, on immune and epithelial cells. Some bacterial signals may promote the inflammatory conditions needed for effective tumor immunity, while others may support regulatory immune responses that dampen attack. The review discusses peripheral regulatory T cells, often called Tregs, as one component of this balance. A microbiota that favors excessive immune suppression could weaken the systemic benefits of radiotherapy; a community that supports properly controlled immune activation might help convert local radiation damage into a broader antitumor response.
The same microbial ecosystem can become harmful when radiation disturbs it. Dysbiosis—a shift in the composition or function of the microbial community—may reduce beneficial bacteria, permit the expansion of inflammatory species and alter the production of protective metabolites. Damage to epithelial cells can weaken tight junctions, the protein structures that seal neighboring intestinal cells together. Proteins such as zonula occludens-1, or ZO-1, help maintain this barrier. When tight junctions deteriorate, bacteria and bacterial products can cross more easily into the underlying tissue, stimulating immune cells and amplifying inflammation. This cycle can produce diarrhea, abdominal pain, bleeding and other symptoms associated with radiation enteritis. The review suggests that barrier breakdown is not merely a side effect of treatment but part of a feedback loop in which microbial imbalance intensifies tissue injury and prolonged inflammation makes recovery more difficult.
Certain bacteria may also promote cancer biology and undermine radiation response. The review refers to enterotoxigenic Bacteroides fragilis, or ETBF, which produces a toxin known as BFT. Such microbial factors can alter epithelial signaling, encourage inflammation and contribute to a tumor-promoting environment. Other bacterial products, including trimethylamine N-oxide, indole-3-propionic acid and cyclic di-AMP, may influence vascular, immune or epithelial pathways in ways that differ according to the local disease state. The consequences are therefore not reducible to a simple division between “good” and “bad” bacteria. The same organism may behave differently depending on diet, medication exposure, oxygen levels, tumor location and interactions with neighboring microbes. This complexity helps explain why broad claims that any probiotic will improve radiotherapy are premature.
The review examines several strategies intended to manipulate the microbiome during cancer treatment. Probiotics could introduce or support selected microorganisms, while prebiotics provide substrates that encourage the growth of potentially beneficial species. Antibiotics might suppress bacteria that promote inflammation or resistance, but they can also eliminate organisms needed for immune stimulation and metabolic balance. Fecal microbiota transplantation, or FMT, attempts to transfer a community of microorganisms from a screened donor to a patient. In principle, microbial interventions could be timed before, during or after radiotherapy to enhance tumor killing while preserving normal tissue. In practice, each approach carries risks. Probiotic strains may fail to colonize, antibiotics can produce profound and unpredictable ecological changes, and FMT may transmit pathogens or unwanted biological traits despite screening. None should be viewed as a universally safe supplement to radiotherapy.
A major obstacle is the extraordinary individuality of the human microbiome. Two patients with colorectal cancer may differ in bacterial species, microbial genes, metabolite production, diet, immune status and previous exposure to antibiotics or chemotherapy. Even within one patient, the microbiota can change from day to day. Stool samples, commonly used for analysis, may not accurately represent microbes living next to a tumor or embedded in the intestinal mucus. Radiation dose, treatment schedule and tumor location add further variables. The authors therefore call for high-quality clinical trials with standardized methods for collecting samples, defining dysbiosis, measuring metabolites and evaluating both tumor control and toxicity. Future studies may combine microbial sequencing with metabolomics, immune profiling and clinical imaging to identify signatures that predict response. Until those studies are completed, microbiome modulation remains a promising research frontier rather than an established radiotherapy prescription.
The central message of the review is that radiotherapy should not be understood as an interaction between radiation and cancer cells alone. It is delivered within a living ecosystem in which bacteria can influence DNA damage, immune surveillance, epithelial repair and inflammation. Metabolites such as SCFAs and methylglyoxal may amplify the destruction of malignant cells, while dysbiosis and barrier failure can intensify the collateral damage to healthy intestine. Translating this insight into treatment will require precision rather than simplistic microbial engineering: the goal may be to adjust specific organisms, functions or metabolites for a particular patient at a particular stage of therapy. If clinical research can establish which microbial changes are causal and which are merely associated with treatment outcomes, the gut could become a controllable component of colorectal cancer care—helping radiotherapy hit tumors harder while sparing the tissue that makes the treatment so difficult to endure.
Subject of Research: The role of gut microbiota and microbial metabolites in modulating radiotherapy efficacy and toxicity in colorectal cancer
Subject of Research: Medicine
Article Title: The gut microbiota: a key modulator of radiotherapy efficacy and toxicity in colorectal cancer
Article References: The gut microbiota: a key modulator of radiotherapy efficacy and toxicity in colorectal cancer — canonical source Original publication
Image Credits: AI Generated
DOI: 10.1186/s12967-026-08633-w
Keywords: gut microbiota, colorectal cancer, radiotherapy, radiosensitivity, radiation enteritis, microbial metabolites, probiotics, fecal microbiota transplantation
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