**5. Concluding remarks**

Considering CRC microenvironment and the functionality of intestinal microbiome, it has a high importance as a risk factor or can be directly associated to CRC. Chronic inflammatory processes driven by dysbiosis can affect all stages of tumor development by compounds that can damage DNA, for example, reactive oxygen species (ROS) promoting CRC development [91]. During an inflammatory response, this microenvironment generates ROS and reactive nitrogen intermediates (RNIs) that lead to deleterious DNA promoting carcinogenesis damage or activating pivotal signaling pathways for adenoma formation and growth [86]. In a study investigating CRC in mouse model of tumorigenesis, it was demonstrated that long-term inflammation-mediated breakdown of protective intestinal barriers promotes the production of some inflammatory cytokines such as IL-17 and IL-23 that lead to tumor growth by facilitating bacterial translocation and consequently microbial products that trigger tumorigenesis resulting in adenoma invasion [92]. Blooms of enterobacteria also seem to play a role in CRC as indicated by metagenomic studies of luminal microbiota of inflamed Il10−/− mice reveal that *E. coli* can promote cancer activity modulating tumor development

**Table 1.** Recent metagenomic studies in clinical investigation of intestinal inflammatory diseases.

**Main findings Disease Sampling description Ref**

CRC Tissues samples from 108

CRC Tissue was sampled from 158 CRC patients, 24 adenoma patients and 14 normal colon

CRC 16S rRNA amplicon sequence raw datasets from 12 studies

controls

patients in stages I–IV of CRC with different prognosis

[109]

[110]

[111]

*F. nucleatum*, *B. fragilis* and *F. prausnitzii* could be identified as

*B. fragilis* and *F. prausnitzii* correlated with patient's survival

CRC microbiome is stage-specific and appears to evolve with disease progression. Enrichment of organisms including *Bacteroides fragilis*, *Fusobacterium nucleatum* and underrepresentation of *Bacteroides vulgatus*, *Bacteroides uniformis* and

*F. nucleatum* presented higher abundance in non-survival

Enrichment of oral pathobionts in poor prognosis tumors and cancers: *Parvimonas micra*, *Porphyromonas gingivalis* and

Multiple fusobacteria members did not correlate with CRC. Enriched *F. nucleatum*, *F. necrophorum*, *Leptotrichia trevisanii*, *B. fragilis*, *Parvimonas micra*, *Peptostreptococcus* 

Low levels of *F. varium* and *Cetobacterium somerae* in CRC

useful prognostic biomarkers for CRC

66 Metagenomics for Gut Microbes

*Faecalibacterium prausnitzii*

*stomatis* and *Gemella morbillorum*

*Prevotella* spp.

in CRC

group

In a study with fecal samples from 74 patients with CRC, the microbiome was enriched in *Fusobacterium nucleatum* and *Peptostreptococcus stomatis*. Moreover, co-occurrence of these taxa with the other two, *Parvimonas micra* and *Solobacterium moorei,* was found. Interestingly, *P. micra* and the Gram-negative *F. nucleatum* can induce inflammatory responses by binding to lipopolysaccharides from Gram-negative bacteria. Furthermore, *F. nucleatum* was shown to increase intestinal tumorigenesis through recruitment of infiltrating immune cells and through activation of β-catenin signaling [90]. In another study, in which the role of *F. nucleatum* in CRC was investigated by metagenomic analyses of 511 colorectal carcinomas from Japanese patients, it was identified a significant increase in the occurrence of

once host inflammation has been established [87, 93].

the bacteria [89].

These recent metagenomic studies reiterate that microbiome intrinsic factors from particular communities and lifestyle are extremely important and should be considered for future development of novel therapies for IBD. Despite of all efforts to accurately unravel the microbiome role in the gut and its relationship with gut inflammation through metagenomic sequencing approaches, it seems that more complex mechanism might be involved in dysbiosis linked to chronic inflammatory diseases, such as IBD and CRC. In this context, further advances in the area are required to achieve a more precise definition of gut microbiome role, which could allow investments for the search of more effective therapies.
