Bacteria are present in a large number of human tumors. Their origin is not fully elucidated, but most likely the hypoxic and immunosuppressive tumor microenvironment is favorable for their proliferation and colonization. The development of new approaches and methods for the detection and identification of intratumor bacteria, such as 16 rRNA sequencing, FISH and metagenomics, has allowed us to realize that bacteria are specific according to tumor type. Their proportion and composition in different tumors is important for the evaluation of possible influences on tumorigenesis. In particular, they can negatively affect the health of patients by creating an inflammatory microenvironment that promotes the division of cancer cells, by inducing changes within tumors through the release of bacterial metabolites, by activating oncogenic pathways and inhibiting the antitumor immune system, and last but not least by promoting resistance to chemotherapy. But they also have the ability to increase the immune response of patients and thereby reduce the growth and development of new tumors. This knowledge is particularly useful in the development of new approaches to cancer treatment. By simultaneously using probiotics or antibiotics with traditional therapy, we can transform the composition of bacteria and increase the response to treatment. Another option is to use genetically modified bacteria for bacterial cancer treatment or as living systems for the targeted delivery of therapeutic molecules to tumor sites. However, this is not yet fully feasible due to lack of knowledge and inconsistencies between data from research studies.
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