Bacterial Involvement in Progression and Metastasis of Colorectal Neoplasia

SIMPLE SUMMARY: Increasing evidence suggests that bacterial infection not only promotes carcinogenesis in primary colorectal cancer, but also affects metastatic progression and organ selectivity through modification of the microenvironment at primary and secondary tumor sites. The metastatic cascade is the process by which neoplastic tumors potentiate cancerous spread to distant organs, and evidence suggests that this process is provoked in the setting of bacterial infection. Biofilm formation, paired migration, and quorum sensing are processes by which bacteria self-signal, recruit, and effectively establish a pre-metastatic niche at distant sites, rendering a suitable environment for tumor cell survival and proliferation. ABSTRACT: While the gut microbiome is composed of numerous bacteria, specific bacteria within the gut may play a significant role in carcinogenesis, progression, and metastasis of colorectal carcinoma (CRC). Certain microbial species are known to be associated with specific cancers; however, the interrelationship between bacteria and metastasis is still enigmatic. Mounting evidence suggests that bacteria participate in cancer organotropism during solid tumor metastasis. A critical review of the literature was conducted to better characterize what is known about bacteria populating a distant site and whether a tumor depends upon the same microenvironment during or after metastasis. The processes of carcinogenesis, tumor growth and metastatic spread in the setting of bacterial infection were examined in detail. The literature was scrutinized to discover the role of the lymphatic and venous systems in tumor metastasis and how microbes affect these processes. Some bacteria have a potent ability to enhance epithelial–mesenchymal transition, a critical step in the metastatic cascade. Bacteria also can modify the microenvironment and the local immune profile at a metastatic site. Early targeted antibiotic therapy should be further investigated as a measure to prevent metastatic spread in the setting of bacterial infection.