**1. Introduction**

The GI tract is a complex ecosystem, populated by a large variety of bacteria, fungi and viruses that together form the intestinal microbiome. A surprising amount of local and systemic bodily functions are affected by the composition of the microbiome and its produced metabolites. This includes the generation of energy, metabolism of dietary components and synthesis of vitamins as well as regulation of immune responses, behaviour and mood. Perturbations of microbial populations, commonly referred to as dysbiosis, have been associated with a large number of diseases, such as inflammatory bowel disease [1], diabetes [2], obesity [3], autism [4], depression [5] and colorectal cancer [6, 7]. Understanding the reciprocal relationship between the microbiota and immunity has received great attention as it is becoming increasingly clear that inflammatory processes underlie many pathologies. The complexity of microbiome-immune interactions is staggering as not only the presence or absence of bacterial species shape immunity, but metabolites produced and modified by bacteria have a direct effect on the immune system's ability to react to infectious and non-communicable diseases [8].

Microbial metabolites, as the sum of products modified and synthesised by microbiota, can be a useful tool to understand microbiota-driven immune modulation when analysis of bacterial lineages proves difficult. Diversity and abundance of microbial communities varies greatly amongst healthy individuals, whereas metabolic pathways are conserved and stable [9, 10]. Therefore, assessing changes of metabolic pathways and how they affect immunity may provide crucial insights into the role of the GI microenvironment in health and disease. Microbial metabolites are commonly divided into three categories, (1) metabolites produced by bacteria, derived from host products; (2) metabolites modified by bacteria, derived from host products; and (3) metabolites synthesised by bacteria directly.

In the following sections, we will briefly describe GI cancers and components of the GI tract that shape the tumour microenvironment. Furthermore, we will discuss the evidence for connecting changes in the microbiome and its metabolites with carcinogenesis and the role of bacterial metabolites in shaping immunity and in particular anticancer immunity.
