Microbiome and microbial products as immunomodulators: Implications for gastrointestinal cancers

The Hippocratic adage "all disease starts in the gut" remains pertinent as research uncovers how the gut microbiome and its products influence immunity and gastrointestinal cancers. The immune system's remarkable paradox lies in its ability to distinguish and tolerate trillions of beneficial microbes while defending against harmful pathogens to maintain health. Disruptions to this delicate balance, known as dysbiosis, contribute to chronic inflammation and create a favourable tumour development and progression environment. Microbial metabolites, such as- short-chain fatty acids and secondary bile acids further modulate inflammation, gut barrier integrity, and immune checkpoint pathways. Emerging microbial biomarkers show promise in diagnosis and prognosis and compete with traditional clinical factors. Innovative therapeutic strategies are being explored to harness the microbiome's immunomodulatory potential. Integrating microbiome profiling into personalised medicine offers new opportunities to prevent, detect, and treat gastrointestinal cancers, shifting the microbiome from a passive disease marker to an active agent that modulates and influences the disease. This chapter discusses the emerging understanding of microbiomes and microbial products in gastrointestinal cancers, highlighting diagnostic advances and novel therapeutic approaches that leverage microbial interactions to improve patient outcomes.