Table 1.

An overview of the microbiota in the cancerous tissue, and their role in tumorigenesis and tumor progression across different cancer types.

Cancer type	Name of the identified tumor-associated microbiome	Proposed mechanism
Lung cancer	Veillonella (Yan et al., 2015; Lee et al., 2016; Tsay et al., 2018)	N/A
	Streptococcus (Cameron et al., 2017; Tsay et al., 2018)	N/A
	Acinetobacter (Cameron et al., 2017; Gomes et al., 2019)	N/A
	Capnocytophaga (Liu et al., 2018)	N/A
	Thermus and Legionella (Yu et al., 2016)	N/A
	Megasphaera (Lee et al., 2016)	N/A
	Granulicatella adiacens, Enterococcus, and Escherichia coli (Cameron et al., 2017)	N/A
	Brevundimonas, Propionibacterium, and Enterobacter (Gomes et al., 2019)	N/A
	Prevotella and Rothia (Tsay et al., 2018)	N/A
	Acidovorax (Greathouse et al., 2018)	N/A
Colorectal cancer	Fusobacterium nucleatum (Castellarin et al., 2012; Kostic et al., 2012, 2013; Rubinstein et al., 2013; Bullman et al., 2017; Yu et al., 2017; Garrett, 2019)	Binding of F. nucleatum adhesin molecule to the cell surface motifs on cancer cells or immune cells, which leads to the downstream oncogenic or immunosuppressive signaling (Garrett, 2019).
	Enterotoxigenic Bacteroides fragilis (ETBF) (Dejea et al., 2018)	Tumor-coating ETBF recruits other bacteria as well as immune cells to the tumor site and boosts IL-17-mediated inflammation (Dejea et al., 2018).
	Escherichia coli expressing the genomic island polyketide synthase (pks+ E. coli) (Arthur et al., 2012; Dejea et al., 2018)	pks+ E. coli-derived colibactin alkylates DNA and produces DNA adducts, resulting in DNA damage in colonic epithelial cells (Wilson et al., 2019).
	Bifidobacterium (Shi et al., 2020)	N/A
Pancreatic ductal adenocarcinoma	Proteobacteria (Geller et al., 2017; Pushalkar et al., 2018)	Proteobacteria lead to T cell anergy in a Toll-like receptor-dependent manner, accelerating tumor progression (Pushalkar et al., 2018).
	Malassezia globose (Aykut et al., 2019)	Contributes to tumorigenesis, tumor growth, and gemcitabine resistance via mannose-binding lectin-C3 axis (Aykut et al., 2019).
	Pseudoxanthomonas, Saccharopolyspora, and Streptomyces spp. (Riquelme et al., 2019)	N/A
Esophageal cancer	Fusobacterium nucleatum (Yamamura et al., 2016)	Contributes to tumor infiltration of Treg lymphocytes in a chemokine (especially CCL20)-dependent fashion, promoting aggressive tumor behaviors (Yamamura et al., 2016).