Our observations over the course of the study suggest that we have discovered a novel pattern to the human CRC microbiome among Han Chinese. According to the bacterial driver-passenger model for CRC put forward by Tjalsma et al (2012) , among 50% of Chinese CRC patients in this study, the over-represented Roseburia bacteria at tumor sites should in fact be considered as ‘passenger bacteria’ for CRC (previously known in other populations, but not previously observed in Chinese) and the over-represented Microbacterium and Anoxybacillus bacteria away from tumor sites—i.e., adjacent non-malignant tissue—as ‘driver bacteria’ for CRC (novel among all populations).
Consistent with several previous reports [3, 4, 11–13], we also found an increasing trend of Fusobacterium spp. in tumors among 87.5% of patients, though we did not discover a significant increase. Across several of the different, previously studied populations including Europeans, Americans, and Asians (in particular, Chinese and Vietnamese), a similar pattern was observed; such a consistent overrepresentation of Fusobacterium spp. in tumor tissue suggests that there exists a common mechanism of gut microbial disorder connected with CRC. This finding implies that it is crucial to begin defining an underlying association of the gut passenger bacteria Fusobacterium with an increasing risk of CRC for most populations, despite the lack of relative reports for many different populations to date.
We also found another gut bacterium, Roseburia, is potentially associated with an increased risk of CRC, due to the overrepresentation of Roseburia in tumor tissue among Chinese, as we noted in this study, and as previously reported among Dutch . This finding is contrary to the study done by Wang et al study on another Chinese population  that compared the difference of stool flora between CRC patients and healthy subjects. Among the Chinese, the distinct discovery of Roseburia and its potential association with CRC may be due to different sample types, as another study done by Chen et al indicated a different microbial structure between the intestinal lumen and cancerous tissue in Chinese CRC patients. The microbial structure difference between the intestinal lumen and mucosa tissue in healthy subjects was further confirmed by Eckburg et al. Additionally, two factors —diet and genetics—may have minor effects on the differentiation of Roseburia for CRC among Chinese, as we observed consistent overrepresentation of Roseburia in tumor tissues between the Chinese and Dutch, two populations with higher divergence of diet and genetics than within the Chinese population. Accordingly, the potential role of candidate gut passenger bacteria Roseburia should be emphasized more heavily during the occurrence of CRC, regardless of the population (e.g. Chinese or Dutch) and further investigated.
Similar to the relationship between CRC and gut bacteria Fusobacterium and Roseburia described above, the ‘passenger bacteria’ role of Bacteroides for CRC was supported by studies on the Chinese , Dutch , and French . And the ‘driver bacteria’ role of Bacteroides was likewise confirmed by two other studies that examined a similar pattern among the Spanish, American and Vietnamese [3, 4]. Furthermore, Wu et al confirmed that gut bacteria Bacteroides fragilis enables the promotion of colon tumorigenesis . Similarly, based on in vivo experiments of mice model, E. coli-induced colitis is a driving factor of colorectal cancer , and the ‘passenger bacteria’ role of Escherichia species for CRC among the Chinese was also implied by the study done by Wang et al.. Moreover, besides the common pattern of the human CRC microbiome represented by gut passenger bacteria Fusobacterium within and between populations, there exists a diversified pattern in the human CRC microbiome due to three possible factors. One may be due largely to high variation of the normal human gut microbiome , potentially associated with diet [17, 18], age [18, 19], sample type (mucosa or stool) , host genetic factors , or other factors, such as antibiotic abuse . Another factor may result from the different stage of tumor progression that is randomly selected by different studies, as the CRC microbiome variance may be temporally associated with developing tumors . The last possible factor is that most findings to date have only been derived from bacterial 16S rRNA-based analyses, though some evidence from a metagenomic approach with a markedly more powerful ability to decipher the landscape of human CRC microbiome are intriguing [3, 4]. Further studies of this kind will be helpful in confirming and elucidating the potential associations we have outlined in the present study.
In summary, in the present study we presented some initial findings the lead towards a deeper and more comprehensive view of the human CRC microbiome. The existing findings are suggestive of further research, and underscore the necessity of borrowing from both high-throughput meta-genomic or transcriptomic data and (animal) model experiments that will better define and validate the association of high-risk microbial populations with occurrence of CRCs across different populations.