Table 2.

The role of gut microbiota in IBD and CRC.

Bacteria	Role in IBD	REF	Role in CRC	REF
Bacteroides fragilis, Escherichia coli, Enterococcus faecalis, and Streptococcus bovis.	-their levels are pathogenic for IBD.	[4]	-their levels are related to the development of CRC.	[4]
Bacteroides fragilis and Escherichia coli	-are related to tissue damage consequent to the activation of STAT3 and IL-17; -persistent STAT3 activation contributes to increase inflammation.	[103]	-persistent STAT3 activation in cancer cells is linked to cell survival, angiogenesis, and metastatic processes, contributing to increase tumorigenesis.	[103]
Enterococcus faecalis	-is responsible for the production of the ROS; -IL-10 knock-out mice have higher susceptibility to develop IBD induced by Enterococcus faecalis (animal model).	[104] [105]	-IL-10 knock-out mice have higher susceptibility to develop rectal dysplasia and cancer induced by Enterococcus faecalis (animal model);	[105]
Streptococcus bovis and Bacteroides fragilis	-are associated with the production of pro-inflammatory and pro-angiogenic cytokines, such as IL-6, IL-8, and IL-17.	[104]
Lactobacilli	-modification of the gut microbiota in IL-10 knock-out mice by these bacteria was related to a reduced prevalence of intestinal inflammation (animal model).	[106]	-modification of the gut microbiota in IL-10 knock-out mice by probiotic was related to a reduced prevalence of CRC development (animal model).	[106]
Salmonella/Campylobacter	-episodes of Salmonella/Campylobacter gastroenteritis have been associated with increased risk of developing IBD (human study); -alterations in the TLR4 gene may predispose to these Gram-negative bacteria infections and generic increased susceptibility to enteric infections (human study).	[107] [107]
Clostridium, Lactobacillus, Faecalibacterium, and Bifidobacterium	-their genus seems to play a beneficial role against IBD; -various Clostridia strains can promote Tregs development in colonic mucosa and thus may protect from colitis.	[108]
Faecalibacterium prausnitzii	-has been demonstrated to attenuate the severity of intestinal inflammation through the production of metabolites able to stimulate a tolerogenic cytokine profile, the enhancement of the intestinal barrier function as well as the inhibition of NF-κB signaling and IL-8 production.	[109]	-several studies correlate the depletion of the butyrate producer Faecalibacterium prausnitzii with CRC development (human study); -due to its crucial role in maintaining gut and host physiology, the microorganism has been proposed as a biomarker of CRC; -its protective effects are not clear cut-defined since other evidences have shown no depletion of the microorganism in CRC (human study).	[110] [111] [109] [112]
Fusobacterium nucleatum			-is suggested as another predictive and prognostic biomarker for CRC; -the intratumoral presence of this CRC-enriching microorganism correlates with a poor prognosis due to higher microsatellite instability and gene mutation (human study); -promotes tumor progression, metastatization and chemoresistance through its ability to influence tumor cells and several tumor microenvironment components (human study); -the association between Fusobacterium nucleatum and CRC initiation is still unresolved and needs to be further elucidated; -cannot be completely considered as a pro-carcinogenic microorganism since its role depends upon the genetic background of the host, tumor microenvironment and environmental factors.	[113] [113] [114] [114]