There are complex microbial inhabited in human intestine, and the group maintained the stability of intestinal environment and protect the health of human body. These microbial participate in defense and immunity against pathogens, development of intestinal microvilli, and ferment non-digestible dietary fibers, anaerobic metabolism of peptides and proteins, providing energy to the host. A number of studies have shown that the gut microbiome is associated with the occurrence of CAPs. However, most intestinal bacteria cannot be cultured in vitro, the application of high-throughput sequencing technology helped us fully understand how the gut microbiome changed during the development process from healthy to CAP and CRC.
In the 16S rRNA sequence analysis of faeces, the alpha diversity index was not statistically significant between the two groups. Goedert et al. reported similar results of faecal microbiota in CAP patients, but a reduction in the abundance of faecal microbiota was observed in CRC patients. PCoA analysis of weighted UniFrac revealed no obvious aggregation in the CAP group and HC group. The changes in genera in both groups were analysed, and Weissella and Lactobacillus were present in the HC group, although the relative content was not high, but Weissella and Lactobacillus are probiotics. The difference in the two groups indicated that the two genera might have a protective effect on the intestine. In addition, the abundances of Bacteroides and Citrobacter in the CAP group were higher than those in the HC group, which indicated that these two species might play an important role in the pathological process of CAP. Studies have found that Citrobacter can take over the “cell-cell" communication system to trigger colitis in mice, and the elevation of Bacteroides in the faeces of CAP patients has been confirmed . Compared with the bacteria in faeces, the bacteria attached to colonic mucosa were more likely to affect the gene expression of colonic mucosa cells. Based on high-throughput sequencing of biopsy tissue, the alpha diversity of polyps was higher than that of healthy tissue, which indicated that polyps have higher within-habitat diversity than does healthy tissue. This phenomenon of increased diversity also appeared in studies on CRC, which might suggest that increased diversity of the gut microbiome is not a sign of healthy intestines but rather the excessive growth of various harmful bacteria or archaea in adenoma and cancer development. Studies on the faeces and tumour tissues of CRC patients have shown different results for Bacteroides[18–20]. Yu et al. found that the abundances of Proteobacteria and Fusobacteria were high in tumour tissues of CRC patients, but in this study, there were no significant differences in the two groups of bacteria. Many studies have found Fusobacteria enriched in the faeces and tumour tissues of patients with CRC, but there were no significant differences in the abundance of Fusobacteria in faeces or adenoma tissues between CAP patients and healthy controls. The relative content of Fusobacteria in faeces and polyps was low, and it was speculated that the enrichment of Proteobacteria may be related to the degree of tissue abnormality.
In this study, the acetic acid and butyric acid contents in the faeces were higher in the CAP group than in the HC group. Butyrate, as a major source of energy for intestinal epithelial cells, could reduce colonic inflammation, induce apoptosis, inhibit tumour cells and prevent CRC development. The antiproliferative and anticancer properties of butyrate have been demonstrated and are probably attributable to the effect of high concentrations of butyrate as a histone deacetylase inhibitor (HDACi). However, Bultman et al.  believed that butyrate is a causative factor of CRC, and a study on APCMin/+MSH2−/− mice fed butyrate showed that the amount of butyrate administered is positively correlated with the formation of polyps in mice, which might be due to the stimulation of hyperproliferation of the gut microbiota and cell transformation of mouse intestinal epithelial cells through metabolites. The formation of polyps at low concentrations stimulates the proliferation of colonic epithelial cells. These opposing effects of butyrate have been called the "butyrate paradox". Although the propionic acid content was not significantly different between the two groups, the propionic acid content in the CAP group was increased, and Bacteroides, which is a major contributor to propionate synthesis, was significantly abundant in the CAP group. Due to the different positions and conformations of the conjugated double bonds, there were multiple isomers of conjugated linoleic acid and the main ones being c9,t11-CLA and t10,c12-CLA , these two isomers played different roles in anticancer and anticardiovascular disease activity. Here, t10,c12-CLA was found to be increased in the faeces of the HC group, but the difference in c9,t11-CLA content between groups was not statistically significant. CLA has functions such as reducing body fat, restricting tumour development, preventing cardiovascular disease and improving immunity. As a fatty acid that protects the intestine, its antitumour properties in vitro and in vivo have been widely recognized. Among the isomers, t10,c12-CLA has functions of reducing body fat, lowering triglyceride content and inhibiting adipocyte differentiation, and t10,c12-CLA was found to be more effective in inhibiting tumours than were other isomers. The effects of inhibiting tumour cell growth were positively correlated with its concentration, and c9,t11-CLA played an important role in immune regulation[28–30]. In addition, t10,c12-CLA showed a significant decrease in the CAP group, and it was considered that the decrease in c9,t11-CLA content in faeces might increase the risk of intestinal adenomatous polyps. A high-fat diet strongly stimulates bile acid production, and bile acids are converted to secondary bile acids (DCA and LCA) after structural modification of bacteria with 7α-dehydroxy activation in the gut. DCA is the most typical secondary bile acid. Secondary bile acids promote the proliferation of intestinal epithelial cells, induce apoptosis and mutation, and promote cancer progression. There were no significant differences in the levels any bile acids in our study, but the content of DCA was higher in the CAP group. Lu et al. found that CDCA and DCA showed a significant increase in faeces only in CRC patients, but no significant differences were observed in CAP patients. The gut microbiota converts primary bile acids into secondary bile acids, suggesting that the gut microbiota can affect the composition of secondary bile acids, while changing the profile of secondary bile acids could reshape the composition of intestinal bacteria. Although there was no significant difference in DCA or CDCA between groups, the abundance of Bacteroides, which has bile acid-resistant characteristics, was positively correlated with fat and protein intake[24, 34, 35], and the abundance of Bacteroides showed a significant increase in the CAP group.
There are researches indicate the relationship between gut microbiota and metabolites in intestinal track. The ability of gut microbiota to produce metabolites such as butyrate, secondary bile acid and CLA can vary the regulation of gut environment, as has been shown in response to diet. This study aimed to investigate if there were gene expression differences between CAP patients and healthy volunteers. In analysis of DNA from faeces, the expression of butyrate-producing bacterial genes in the CAP group was significantly lower than that in the HC group, but there was no significant difference of DNA between the two groups. The result indicated that the abundance of butyrate-producing bacteria in the faeces of CAP patients was decreased, while the content of butyric acid in the CAP group was higher than that in the HC group. Therefore, the concentration of SCFAs in faeces does not fully reflect the concentration of SCFAs produced by gut microbiota fermentation; thus, the intestinal health effects need to be carefully considered. SCFAs can effectively reduce the pH of the intestine, promote the glycolysis of food in the intestine and reduce the absorption of carcinogens, which can reduce the risk of CRC. The differences in secondary bile acid-producing bacteria and CLA-producing bacteria were not statistically significant in this study. Mullish et al.  have found that baiCD operon was not in all bacteria with 7-α-dehydroxylating ability, which has been considered to an important process for the formation of secondary bile acids in faeces. Certain species of Bifidobacteria in the gut, as natural colonizers, were capable of converting linoleic acid to c9,t11-CLA, t10,c12-CLA and small amounts of t9,t11-CLA . There was no statistically significant difference in c9,t11-CLA content in faeces between the two groups, and it was considered that the abundance of Bifidobacteria that produce c9,t11-CLA was not significantly different in the two groups.
Metabolomics provide a qualitative and quantitative method of metabolites in analysis which can complete analysis with microbiology. Metabolites (small molecules < 1500 Da) are intermediates or end products of cellular metabolism, which can be produced directly by the host organism, or can be derived from various other external sources such as diet, microbes, or xenobiotic sources. Biological systems are complex and analytical limitations, it is not possible to identify all the metabolites present in a specimen. Researches about metabolites and diseases indicate that diabetes, cardiovascular disease and heart failure[42, 43], autism and anxiety. With technology and researches progressing, more and more evidence would show that specific species of gut microbiota and metabolites play an important role in disease, which would be biomarker for disease diagnose in future.
In this study, the detection of gut microbiota and metabolites indicated that there still existed something insufficient. First of all, the structure of sample disease stage were unbalanced, the proportion of high-grade intraepithelial neoplasia were lower than others samples. It is necessary to expand sample size, elevate the proportion of high-grade intraepithelial neoplasia. Second, the direct interaction between gut microbiota and metabolites, that the proportion of specific gut microbiota and its related metabolites content also need further research. Third, the effect of food and antibiotics intake on intestinal microenvironment, and its influences on disease occurrence and progression is a promising study. Nowadays, with technology developing, the detect limitation would be solved and have multiple vision on correlation between gut microbiota and metabolites in future.