**5. The biliary microbiome**

According to traditional thinking, the biliary tract has always been considered sterile. This is because of the anti-microbial properties possessed by bile which affects the bacterial membrane and DNA [67]. However, inflammatory conditions of the biliary tract, like acute cholecystitis and cholangitis have frequently cultured bacterial colonies commonly found in the human gut; the common organisms being, *Escherichia coli*, Enterobacter, Pseudomonas, and Citrobacter spp. [68]. This can be explained by the pathophysiology of these diseases, which is, biliary obstruction and gut bacterial translocation. However, recent studies have indicated that even under nonpathogenic circumstances, the human bile comprises a rich diversity of microbial flora which is actively involved in the regulation of the size and composition of the bile acid pool as well as the metabolism of bile acids [69, 70]. However, this normal biliary microbiome mainly included Firmicutes, Actinobacteria, Proteobacteria, and Bacteroidetes which were also found in the human gut [70, 71]. This can partly be explained by the close association of the human gut and the biliary tract and the involvement of enteric bacteria in enterohepatic circulation but the evidence is largely limited to animal models and an ideal human experimental model is required [72].

Dysbiosis of the gut bacteria has been implicated in the development and progression of various cancers, including gastric [73, 74], colorectal [75], and oral cancers [76, 77], however, their association with causation or progression of the hepatobiliary cancers is still in question. The natural synergy that exists between the bile acid metabolism and the biliary microbiome reaffirms the proposition that biliary microbial dysbiosis may lead to various biliary tract diseases including gallstone formation and the development of cancer.
