Table 1.

The effects of microbes, bacterial components and their metabolites in pancreatic cancer.

Item	Content	Description	Mechanism	References
Bacterial components or metabolites	Lipopolysaccharides	An important cytoderm component of gram-negative bacteria	Interact with several Toll-like receptor signaling pathways with a distinct structural composition from other bacterial taxa	Backhed et al., 2003
	Lipoteichoic acid	A key virulence factor on the gram-positive bacteria surface	Trigger the over-secretion of proinflammatory factors by binding to CD14 or TLR2	Hermann et al., 2002
	Deoxycholic acid	A kind of secondary bile acid generated by 7α-dehydroxylating bacteria from a high dietary fat intake	Accelerate the senescence-associated secretory phenotype and the progression of intestinal cancer via promoting DNA damage and genome instability and activation of the EGFR ligands amphiregulin	Saretzki, 2010; Louis et al., 2014; Nagathihalli et al., 2014
	Short chain fatty acids	Fermented dietary fiber in intestinal tract, including acetate, propionate, and butyrate	Stimulate the secretion of gut peptides involved in food intake or glucose metabolism	Vatanen et al., 2018
			Butyrate inhibits histone deacetylases via interfering histone modifications and transcriptional regulation	Cani and Jordan, 2018
			Propionate decreases the abundance of mucosal-associated invariant T cells and Treg guarding inside the intestinal lamina propria	Cani, 2018
	Cytolethal distending toxin	Produced by proteobacteria	Participate in genetic alterations and induce formation of endoreduplication or hyperploidy even in the absence of cell division	Nougayrede et al., 2005
	Cyclomodulins	A growing family of bacterial molecules	Cause carcinogenesis through the active interference with host cell cycle	Nougayrede et al., 2005
	Cytotoxic necrotizing factor	A prevalent virulence determinant exclusively confined to E. coli phylogroup B2	Lead to the uncontrolled proliferation of cancer cells	Nougayrede et al., 2005
Certain typical bacteria	Enterobacteriaceae	Natural inhabitants in the human intestine implicated in intestinal and extraintestinal illnesses	Promote proliferation by PPAR-γ that requires higher oxygen available for the microbiota at the proximal mucosa	Philipson et al., 2013
	Enterococcus faecalis		Aggravate chronic pancreatitis and damage pancreas tissue by the stimulation inflammatory cytokines	Maekawa et al., 2018
	H. pylori	An initiating factor of kinds of gastrointestinal cancer	Increase the risk of pancreatic cancer relating to gastric ulcer via the greater endogenous nitrosation and the inflammatory response to ulcer development and healing process	Bao et al., 2010
	Porphyromonas gingivalis	The most prevalent oral microorganism for periodontal disease	Its associated serum level of IgG is positively related to the risk of PC	Ahn et al., 2012
	Fusobacterium spp	A group of anaerobic bacterium colonizing oral cavity	Remain malignant potential in the development of pancreatic cancer with the 8.8% presence	Mitsuhashi et al., 2015
	Bifidobacteria	The dominant bacterial populations in the gastrointestinal tract interacted in maturation of the immune system and use of dietary components	Induct tumor-specific T cell and increase CD8 (+) T cell numbers in the tumor microenvironment combined with anti-PD-L1 immunomodulator	Sivan et al., 2015
	Bacteroides	One of the most abundant bacterial phyla in the human gut breaking down host dietary and mucosal polysaccharides	Assist Escherichia coli to improve the tumorigenic effectiveness via triggering damage of double-stranded DNA	Cougnoux et al., 2014