The oral microbiome influences tumor development

The mouth hosts a complex microbial community that plays a key role in protecting tissues, maintaining the integrity of the mucous membranes, and regulating inflammatory responses. This flora, the second most important in the body after that of the gut, also contributes to the balance of the immune system, both locally and systemically. However, an imbalance in this microbiome, known as dysbiosis, is increasingly associated with the onset and progression of various cancers, whether they are located in the oral cavity or in other parts of the body such as the colon, liver, or pancreas.

Certain pathogenic bacteria, such as Porphyromonas gingivalis and Fusobacterium nucleatum, are particularly involved in these processes. They promote tumor formation by altering the inflammatory environment, destabilizing the DNA of host cells, or stimulating the creation of new blood vessels around tumors. These microorganisms can also migrate from the mouth to other areas of the body, such as the digestive tract, where they disrupt local defenses and create an environment conducive to cancer development. For example, Fusobacterium nucleatum is often found in large quantities in the tumor tissues of the colon, liver, or esophagus, while high levels of Porphyromonas gingivalis are detected in the saliva of patients with these same cancers.

Oral dysbiosis does not only influence the initiation of tumors; it also plays a role in their progression and prognosis. In cancer patients, the composition of the oral microbiome is often altered, with a decrease in microbial diversity and a change in the abundance of certain species. These changes can worsen chronic inflammation, weaken immune defenses, or even promote resistance to treatments. Thus, low microbial diversity in saliva is linked to a higher mortality rate in people with nasopharyngeal carcinoma, while greater microbial richness in the healthy tissues of lung cancer patients is associated with a reduced risk of recurrence and longer survival.

The oral microbiome could also serve as a tool for early detection. Studies show that saliva analysis can distinguish with over 90% accuracy between patients with oral cancer and healthy individuals. Similarly, the salivary microbiome offers good predictive capacity for stomach cancer screening, with performance comparable to, or even better than, traditional tests. A combined approach using both oral and fecal microbiome analysis even improves the detection of colorectal cancer, with a specificity of 95% and a sensitivity of 88%.

The mechanisms by which the oral microbiome contributes to carcinogenesis are multiple. Chronic inflammation, induced by microbial imbalance, creates an environment conducive to cell proliferation, genetic mutations, and the activation of genes involved in tumor formation. Some microorganisms also produce carcinogenic substances, such as acetaldehyde, a byproduct of ethanol metabolism, which damages DNA and promotes the malignant transformation of cells. Additionally, oral bacteria can interact with the host’s immune system by activating signaling pathways that weaken the anti-tumor response or by recruiting suppressive immune cells, such as regulatory T cells, which protect tumors from immune system attacks.

Anti-cancer treatments, such as chemotherapy or radiotherapy, also disrupt the balance of the oral microbiome. They often cause mucositis, an inflammation of the oral mucous membranes that alters microbial composition and promotes opportunistic infections. These changes can worsen local and systemic complications, such as the spread of infections or severe immunosuppression, which can compromise the effectiveness of treatments and patient survival.

Finally, the oral microbiome could become a promising therapeutic target. Approaches aimed at modulating this flora, such as the use of probiotics or targeted antibiotics, could help restore a balance favorable to the fight against cancer. Although clinical evidence remains limited, early studies suggest that these interventions could improve the response to anti-cancer treatments by influencing inflammatory and immune pathways. In the future, a better understanding of the interactions between the oral microbiome and tumors could pave the way for new strategies in prevention, early detection, and personalized treatment.

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Study Citation

DOI: https://doi.org/10.1007/s44178-026-00270-z

Title: Research advances in the correlation between oral microbiota and tumors

Journal: Holistic Integrative Oncology

Publisher: Springer Science and Business Media LLC

Authors: Xiaoxuan Liu; Shan Liu; Zhi Guo