Introduction: About 20% of human cancers are linked to infection by viruses, bacteria, or parasites.
Microbiota colonize exposed body tissues (e.g., gastrointestinal tract, skin, lungs, female genital tract, and urogenital tracts) and unexposed sites (e.g., breast). Persistent bacterial infection in the host leads to the development of multiple diseases. They are implicated in the pathogenesis of various complex diseases, including diabetes, atherosclerosis, autoimmune diseases, Alzheimer’s disease, and malignant diseases. A number of studies have demonstrated the role of bacterial infection in carcinogenesis.
Methods: A systematic search has been carried out for articles published in the PubMed and Elsevier databases by using combinations of different keywords with Boolean operators (AND, OR, NOT), including cancer, bacteria, and tumor microenvironment. In the next step, the full texts of the remaining articles were examined, and after removing the irrelevant ones, the results related to the selected articles were compiled by hand in the final stage and examined.
Results: The study of microbiota in tumorigenesis is primarily focused on lung cancer, colorectal cancer (CRC), breast cancer, gastric cancer, and gynecologic tumors, and so on. Infection with Helicobacter pylori in gastric cancer carcinogenesis was recognized as a class I carcinogen by the World Health Organization (WHO) decades ago. The role of Fusobacterium nucleatum in the development of colorectal cancer has been extensively investigated. The identification of microbiota in multiple tumor tissues reveals that bacterial infection and microbiota are associated with tumor development.
Microbiota promote tumor development, modulate the tumor environment to benefit cancer cells and effect the responses to chemotherapy.
Conclusion: Comprehensive research is warranted to address numerous unanswered questions in the interaction of microbiota and host cells. Part of the bacteria plays a driver function in carcinogenesis, part of the bacteria plays a passenger function. Investigation of the role of the microbiota in cancer development may provide targets for antitumor therapy. Moreover, skewing the microbiota balance may prevent tumor development. Finally, the microbiota composition of tumors may be used as an alternative biomarker for predicting prognosis and response to therapy.