ABSTRACT
The discovery of the microbiome and its exploration through next generation sequencing, metagenomics, studies in germ-free mice, and culture techniques has revolutionized nutrition science. The human body is inhabited by over 14 trillion bacteria with the largest number in the intestines. The gut microbiome bacterial populations measured via fecal analysis are stable over time at higher taxonomic levels in an individual but vary among individuals as the result of dynamic interactions of the bacterial communities with diet, lifestyle, the gut-associated immune system, and intestinal epithelial cells. Changes in diet and lifestyle relevant to cancer prevention and treatment can result in changes in the bacterial metabolism of dietary constituents. In turn those metabolites can affect tumor growth and metastasis. In studies of patients receiving cancer immunotherapy, the microbiome was found to impact the anti-tumor immune responses of patients and may explain the individual differences in response durability. When fecal bacteria from patients are transplanted into germ-free mice with tumor xenografts, the human-derived gut bacteria reproduce the variable responses to immunotherapy noted in humans. Bacteria found in and adjacent to tumors can also promote local tumor inflammation, tumor growth, and tumor progression. The microbiome has been demonstrated to affect numerous cancers including colorectal, pancreatic, breast, and prostate cancer. Studies of the microbiota in cancer have included surveys of tumor tissue and tumor-associated fluids, feces, and urine. The role of the microbiome in immunotherapy and its potential to improve responses to cancer therapy will be reviewed in this chapter. Modulation of the microbiome via diet and dietary supplements has the potential to increase the efficacy of cancer prevention efforts, enhance cancer treatment efficacy, and prevent tumor spread and relapse.
