Faecal microbial biomarkers in early diagnosis of colorectal cancer

Abstract Colorectal cancer (CRC) is ranked as the second most common cause of cancer deaths and the third most common cancer globally. It has been described as a ‘silent disease’ which is often easily treatable if detected early—before progression to carcinoma. Colonoscopy, which is the gold standard for diagnosis is not only expensive but is also an invasive diagnostic procedure, thus, effective and non‐invasive diagnostic methods are urgently needed. Unfortunately, the current methods are not sensitive and specific enough in detecting adenomas and early colorectal neoplasia, hampering treatment and consequently, survival rates. Studies have shown that imbalances in such a relationship which renders the gut microbiota in a dysbiotic state are implicated in the development of adenomas ultimately resulting in CRC. The differences found in the makeup and diversity of the gut microbiota of healthy individuals relative to CRC patients have in recent times gained attention as potential biomarkers in early non‐invasive diagnosis of CRC, with promising sensitivity, specificity and even cost‐effectiveness. This review summarizes recent studies in the application of these microbiota biomarkers in early CRC diagnosis, limitations encountered in the area of the faecal microbiota studies as biomarkers for CRC, and future research exploits that address these limitations.

early diagnostic approaches are instituted as this greatly impacts survival rate. 9,10 O'Connell and his colleagues stated that the early diagnosis of CRC at localized stages (American Joint Committee on Cancer (AJCC) stages 0, I or II) increases the survival rate to >80% but if diagnosed in the late stages, when cancer has metastasized, (AJCC stage III or IV), the survival rate is decreased to <10%. 11 A later documentation also stated that the survival rate of individuals who are diagnosed early or have localized CRC is approximately 90%, but the rate drastically reduces to about 14% in patients with metastasized CRC. 12,13 This is the basis for the recommendation of population wide-screening and prevention programs in several countries.
Screening is done primarily to detect cancer at an early treatable stage, 14 thereby preventing carcinogenesis through adenoma detection and complete excision. A review study by Nguyen & Weinberg also emphasized that as a result of the long process involved in the adenoma-carcinoma progression of CRC, detection of CRC early enough positively impacts survival rates as the adenoma can be excised thus, preventing its progression to carcinoma and patients could be identified even before symptoms begin to manifest. 15 The major aim of any CRC diagnostic method is to reduce the overdependence on colonoscopy, the standard for CRC diagnosis, which poses greater risk and is rather very expensive. 15,16 Besides the fact that colonoscopy presents a reasonable level of discomfort for the patient, it is also invasive, relatively costly and poses some health risks such as post polypectomy, puncture of the colon, intraperitoneal bleeding and the possibility of infection. 7,9,15,17,18 Thus, the need for other detection strategies that are both non-invasive and highly effective. Screening methods with high specificity and sensitivity for adenoma detection will by far enhance chances at detecting curable tumours, thus grossly decreasing the mortality and morbidity rates associated with CRC. 9 Stool tests remain the focal point for non-invasive options in CRC diagnosis. 9 Non-invasive stool-based methods such as the guaiac faecal occult blood test gFOBT and faecal immunochemical test FIT are however not reliable in the detection of adenomas although FIT has now largely supplanted gFOBT due to its improved sensitivity. Additionally, some basic limitations such as high cost and reduced sensitivity in adenoma detection exist in the multitarget stool DNA test which has been reported to possess improved diagnostic accuracy compared to FIT and was approved by the US Food and Drug Administration in 2014 15 for the screening of asymptomatic average-risk CRC persons. 19 Hence, an urgent need for new, non-invasive CRC screening methods with increased sensitivity and specificity, for the detection of adenomas and early-stage CRCs. 13 A biological entity that can be employed to determine the presence or the progression of any particular disease or measure the effects exerted by the treatment given is known as a biomarker.
Several important features make for a good biomarker. Such qualities are high specificity, safety, sensitivity, easy to use as a determinant and practical for ascertaining accurate diagnosis as well as enabling appropriate treatment options. 20 More lives will undoubtedly be saved if biomarkers with the above-mentioned qualities in addition to being cost-effective are discovered and employed in the management of CRC. Thus, in essence, many Scientists are doggedly researching this biomarker issue for CRC diagnosis as it not only guarantees early detection of CRC but will also aid in the development of personalized and targeted treatment for affected individuals.
Certain factors such as diet, age, disease, antibiotics, stress, mode of birth, host genetics and other environmental factors determine the composition and diversity of the gut microbiota while the gut microbiota in turn, influences the health of the host with an effect on genes, proteins and metabolites production. 21,22 In a state of homoeostasis, the gut microbiota is beneficial to the host; however, a dysbiotic gut-state can arise if any perturbations occur in the balance of the gut microbiota. Dysbiosis, a metabolic condition that results from the imbalance in a host's gut microbiota, may result in disruption of the host metabolism and immune function leading to several diseases, including inflammatory bowel disease, autoimmune diseases, diabetes, obesity, irritable bowel syndrome, cardiovascular diseases and even cancer. [21][22][23][24][25][26][27] Many gastrointestinal cancers such as pancreatic, 28 liver, 29 gastric 30 and colorectal cancers 31 have been implicated and various researchers have reported the major role played by dysbiosis in the development of adenoma and CRC. [32][33][34][35] With the aid of highthroughput environmental sequencing techniques, a comprehensive makeup of the microbial ecosystem in healthy and diseased states has been made possible. 36,37 It is in recent years that microbes colonizing the gut environment were implicated as potential biomarkers for CRC screening. Fusobacterium nucleatum, Solobacterium moorei, Peptostreptococcus stomatis and Parvimonas micra which are majorly associated with host-gut microbiome imbalance that occurs in CRC have been demonstrated by several studies as potential biomarkers for early CRC screening. [38][39][40][41] In this review, we discuss the recent updates on the application of faecal microbiota biomarkers in the early diagnosis of CRC and it also highlights current findings in gut microbiota metabolites as biomarkers for early CRC detection. Furthermore, the challenges facing this research area and possible future research questions needing urgent answers that address these challenges were explored. The two main non-invasive methods for the diagnosis of CRC is illustrated in Figure 1 with emphasis on the stool-based biomarkers.

| G UT MI CROB I OME AND COLOREC TAL C AN CER
The anaerobic bacteria makeup of a healthy adult even with subtle disparities that exist in different individuals among various populations are made up of two major key phyla (comprising >90% of all the microbiome bacteria population); the Firmicutes and the Bacteroidetes. 42 These intestinal microorganisms, particularly, the bacteria which are the most studied are primarily involved in modulating host metabolism such as synthesis of certain essential vitamins like K and B, extraction of energy from indigestible carbohydrates, protection of the gut from colonization by enteric pathogenic organisms, and immune system modulation. 43,44 The gut, in turn, provides these organisms with an adequate environment, abundant in nutrients, with the right range of pH and oxygen concentration necessary for their growth and metabolism. 45 An interesting fact is that compared to the level of bacteria in the small intestine, the bacteria levels in the large intestine are estimated to be 12-fold higher and more cancers are found to occur in the colon than in the small intestine. This suggests a possible connection between colorectal carcinogenesis and the microbiome (majorly bacteria) of the gut. 46,47 Several studies have suggested that the complexity of CRC disease stems from the interaction of several factors which result in the disease. Genetic, epigenetic, 15,48 and environmental factors have been implicated. The most important environmental factors that influence CRC have been identified as lifestyle and diet, 49 and dietary patterns are known to affect the gut microbiota in real time. 50 Studies have linked the gut microbiota in the development of intestinal adenoma which could progress to CRC. 51 Conversely, healthy gut microbiota is closely associated with a reduced risk of advanced adenoma. 52,53 There are several proofs that many known risk factors for CRC are chiefly implicated in the structure and function of the gut microbiota. 54 These in turn influence immune responses, host metabolism, changes in cancer-driving genomics and epigenomics, and ultimately, CRC development. For instance, just as increased intake of dietary fibres has been shown to result in the enrichment of such bacteria like Bifidobacterium and Lactobacillus spp., by causing increased production of the beneficial short-chain fatty acids (SCFAs) in the gut via fermentation of these dietary fibres by the bacteria, 8 so also is the high consumption of diets rich in red and processed meat known to be unhealthy. This is as a result of the high sulphur-containing amino acids and inorganic sulphur present in them which leads to an abundance of sulphidogenic bacteria for metabolizing these proteinous foods, yielding such metabolites as hydrogen sulphide, ammonia or polyamines known to produce genotoxic components in the gut. As a consequence, DNA damage in intestinal epithelial cells results in fostering colonic carcinogenesis. 55,56 The gut flora of patients with CRC has been found to be composite of certain bacterial species such as Fusobacterium, Solobacterium, Bacteroides, Peptostreptococcus and Parvimonas. 38 It is such that the diversity and composition of their gut microbiota are significantly different from their healthy counterparts. Studies have shown that these microbial compositions can be detected across different ethnic groups and races. Furthermore, since some of these microbial genes are enriched in early-stage CRC, 38 there exists the potential for their application as biomarkers in early CRC detection.
Because the colonic mucosa is in close contact with the gut microbiota and its metabolites, there is bound to be stimulation of immune responses by these bacteria and this may subject the mucosa to continuous low-grade inflammation and chronic inflammation is an established risk factor for several cancers including CRC. 8 Various other researches have also linked chronic inflammation to various cancer types. 45,57,58 It should be noted that under healthy physiological conditions, the gut bacteria and the host are in a state of homeostasis. However, when the balance of the gut microbiome is altered by certain factors such as alcohol intake, some diet types, or even antibiotics treatment, serious problems could arise. 43 This state of the gut known as dysbiosis has been implicated as a major probable cause of certain diseases such as diabetes type 2, cardiovascular diseases, inflammatory bowel disease and CRC. 50,59 Dysbiosis, a condition that arises from the imbalance of the gut microbiota, has been connected by several researchers to the development of adenomas and CRC. 32,33,[60][61][62] Even though the mechanism by which dysbiosis could result in CRC is yet to be fully understood, chronic inflammation is believed to be a major factor. Concurrent with the metabolic shift in the microbiome of CRC patients, Zeller and colleagues, reported an increased array of proinflammatory and pathogenicity processes resulting from the presence of many Gram-negative bacteria. As a result, F I G U R E 1 Current Non-invasive Stool-based Methods for Early Colorectal Cancer Detection. The two main methods, blood-based and stool-based are centred on the same biomarkers. Stool-based biomarkers, which are our focus, have been clearly represented in the diagram. FIT, faecal immunochemical test; gFOBT, guaiac faecal occult-based test; M2-PK, tumour M2 pyruvate kinase; MFBN1, methylated fibrillin-1; MT-sDNA, multitarget stool DNA test; VIM, vimentin inflammation-induced carcinogenesis ensues due to the lipopolysaccharides present on the outer membrane of these bacteria since they elicit an inflammatory signalling cascade by binding to Toll-like receptor 4 in the epithelial cell 63 , further confirming the tumour-promoting activity of inflammation.
One of the earliest large series studies that showcase this, is the research by Sobhani and colleagues. The study showed the relationship between the microbiota of a healthy human and when in a diseased state. Differences between the gut microbiota of normal individuals and CRC patients were observed. Pyrosequencing was done after 16S rRNA PCR followed by principal component analysis (PCA) using the stool bacterial DNA from patients with CRC and those that have a normal colonoscopy. With this, the researchers were able to determine that more Bacteroides and Prevotella spp.
were represented in individuals with cancer compared to the normal group. Furthermore, they found that IL-17 immune cells were significantly expressed in the mucosa of the cancer patients than in those that have normal colonoscopy indicating that the composition of the gut microbiota could possibly impact the mucosal immune response. 64 In another study by Nakatsu and colleagues, the association of dysbiosis with CRC was investigated by characterizing the mucosal microorganisms of the gut associated with different stages of colorectal carcinogenesis. Using paired samples of adenoma and adenoma-adjacent mucosae, carcinoma and carcinoma-adjacent mucosae, and healthy controls, they determined that correlations of certain bacterial taxa in adenoma, indicate early signs of dysbiosis and in carcinoma, more co-exclusive relationships are found.
They concluded that a well-defined taxonomy of microbial consortium is associated with the occurrence of CRC. 65 Among other observations made by Sun and colleagues in their study using their in-house generated CRC mouse model, an increase in the bacteria inflammatory groups, Bacteriodetes and Porphyromonadaceae led to their proposition that both the interactions of bacteria in a dysbiotic gut and the effects of the metabolites generated on interrelated molecular events contribute to the advancement of colorectal carcinogenesis. 66 Yet, observations made in another study carried out using Apc min/+ mice was that the mice group administered with faeces from CRC patients had more intestinal tumours in comparison with those fed with phosphate-buffered saline PBS or faeces from healthy controls. They further observed that along with increased tumour proliferation, there was decreased tumour cells apoptosis, gut barrier function failure and upregulation of proinflammatory cytokines. Also, an increased abundance of pathogenic bacteria was seen after the feeding with the faeces of CRC patients. This altered gut microbiota not only induced low-grade inflammation but also promoted the progression of intestinal adenoma in Apc min/+ mice. 32 All of these point to the close link that exists between dysbiosis and CRC. However, a conclusion is yet to be reached on whether dysbiosis is a cause or consequence of CRC disease. 51,67,68 The analysis of the microbial composite of faeces could serve as a predictive factor for the risk of developing CRC, because these changes occur even in adenoma stages, hence, potential for their use in early diagnosis of the disease. This relationship between dysbiosis and CRC is illustrated in Figure 2 below. The relative abundance of Clostridium symbiosum was measured by qPCR in colorectal adenoma CRA patients, early/advanced CRC patients and healthy controls, and prediction accuracy compared to F. nucleatum, FIT, and carcinoembryogenic antigen CEA (blood-based biomarker). 73 C. symbiosum was found to perform better than all the other biomarkers employed in the study as a significant stepwise increase of the organism's abundance level was observed in CRA, early CRC and advanced CRC patients compared to the healthy controls. 73 This report makes C. symbiosum a prominent promising biomarker for early and non-invasive CRC diagnosis. In the case of CRC, a statistically significant increasing range of F. nucleatum, E. fecalis, Porphyromonas spp. and P. gingivalis biomarkers distinguished early-stage CRC patients, from AP patients and healthy controls individuals. 51 A significantly higher level of Parvimonas micra was found in the faecal samples of CRC patients compared to the normal controls and it was applied alone for distinguishing early-stage CRC patients from the controls. The sensitivity and specificity obtained were 60.5% and 87.3%, respectively, and the combination of the bacterium with other microbial biomarkers (F. nucleatum and colibactin toxin-producing clb A + bacteria) improved the sensitivity. The study further proposes that in combination with these microbial faecal biomarkers, identification of patients bearing 'high risk' microbial patterns indicative of increased cancer risk may be possible with P. micra. 18 Zagato et al. (2020) 74 in their study identified two gut microorganisms; Faecalibaculum rodentium and its human homologue, Holdemanella biformis that were not only anti-tumorigenic but also had a great reduction in their composition during colorectal F I G U R E 2 Link between Dysbiosis and CRC. In addition to the similarity in factors that trigger the onset of both dysbiosis and CRC like a diet that produces genotoxic metabolomics, the two conditions also share many common damages associated microenvironmental factors. While it is not certain whether dysbiosis is the cause or consequence of CRC, it is well established that CRC is characterized by the unique composition of the gut microbiome that can sometimes serve as a metagenomics biomarker. IECs, intestinal epithelial cells; IL-17, interleukin 17; RNS, reactive nitrogen species; ROS, reactive oxygen species; SCFAs, short-chain fatty acids carcinogenesis. Interestingly, as early as 8 weeks (when the tumour growth had started), genera differences were observed between the Apc Min/+ mice and the C57BL/6 wild-type WT littermates which served as the control. The paired-end reads gave a quantitative reduction of F. rodentium which even became more prominent at 12 weeks. In fact, only the reads credited to F. rodentium (among the ten taxonomic units most abundantly represented in the WT mice)

| Faecal microbiota/metagenomics
were found to be strongly under-represented in the Apc Min/+ mice compared to the WT mice. By interrogating a data set of shot-gun microbiome analysis performed in patients bearing advanced colorectal adenomas and other in vitro and in vivo experiments employing CRC cell lines, tumour tissues from CRC patients and mice, they were able to identify human H. biformis as very closely related to the mouse F. rodentium and could be referred to as a human homologue of the bacterium. In essence, the possibility of translating these research findings in the early detection of CRC at the adenoma stage exists. 74 Clos-Garcia and co. integrated data obtained from metabolomics and metagenomics in their study. They reported that the genera Fusobacterium, Parvimonas and Staphylococcus were increased in CRC patients and became more abundant as the disease progressed. 40 Lachnospiraceae family, on the other hand, were reduced (Table 1). These bacteria genera served to clearly distinguish the CRC group from adenoma and control groups. Interestingly, the genera Adlercreutzia was found to be more abundant in the faeces of adenoma patients compared to the control and carcinoma groups suggesting the possibility of its application as a biomarker in early CRC detection. Barely any differences were reported to exist between the microbiome of control and adenoma groups in all the analytical methods employed in the study.
Researches have shown that Adlercreutzia is one of the most prominent bacteria recognized for its equol production from isoflavonoids present in the host's diet. 75 Equol is implicated in the health of the host as it is known to be associated with higher levels of high-density lipoprotein cholesterol and lower levels of dyslipidemia, 76  The gene m3 from Lachnoclostridium spp. has been identified by 70 as a biomarker for early CRC diagnosis. There was a significant increase in the faecal m3 and Fn from healthy controls, through the adenoma, to the carcinoma groups although faecal m3 may be better than Fn in differentiating adenoma patients from controls. This is because at a specificity of 78.5%, sensitivities of 48.3% and 33.8% for adenoma were observed for m3 and Fn, respectively.
Recent meta-analysis studies of faecal metagenomics (using shot-gun metagenome data) in the diagnosis of CRC have tried to validate the replicability of the microbiome biomarkers across various populations. 39,78 Wirbel and colleagues identified a set of 29 main species with significant enrichment in the CRC metagenomes. These identified CRC microbial signatures which were stated to be validated in diverse populations were also established to be present even in the early stages of CRC. This means that not only are they used for CRC detection, but also successful diagnosis of CRC in the early stages can be achieved irrespective of race and this would in essence, grossly reduce the mortality associated with the disease. 39 Fascinatingly, disease-specific signatures of a dysbiotic gut particular to CRC were applied in the study thus eliminating the possibility of including signatures from gut dysbiotic patterns resulting from other diseases such as inflammatory bowel disease and diabetes. 39 Furthermore, Wu and colleagues employed Random Forest Classifier models in their meta-analysis study to assess the CRC-associated gut microbiome changes and the ability of the integrated features that distinguished the adenoma group from both the control and CRC groups. High diagnostic accuracy with AUC of 0.80 and 0.89 was achieved for the adenoma/control group and adenoma/CRC groups, respectively. These markers also showed high diagnostic accuracy in independent validation cohorts and were also ascertained to be adenoma-specific. 79 Table 1 below summarizes some major findings reported in studies on the use of faecal microbial biomarkers as noninvasive tools in the early detection of CRC.

| Metabolomics
Various studies have suggested that both the concerted activities of the gut microbiota and the influence of its metabolome contribute to the aetiology of CRC. 8,69 Thus, the possible application of these metabolites from a CRC-influenced dysbiotic gut relative to that of healthy individuals as biomarkers for early CRC diagnosis emerged.
The major approach used in the gut metabolome study in CRC patients is the ultra-high performance liquid chromatography-tandem mass spectrometry UHPLC-MS technique 40,80 and proton nuclear magnetic resonance spectroscopy ( 1 H NMR)-based metabolomics approach. 81,82 SCFAs, particularly butyrate, have been reported to be anticarcinogenic due to their anti-inflammatory properties 83 (Figure 2).
On the other hand, some gut bacteria have the ability of metabolizing primary bile acids into secondary bile acids which could promote CRC pathogenesis via the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), known to be genotoxic TA B L E 1 Some major findings on faecal microbial biomarkers in early detection of colorectal cancer.

TA B L E 1 (Continued)
( Figure 2). An example is deoxycholic acid and its effects have been captured in various review articles. 8,83 Recent studies have shown that an altered state of the gut microbiota results in the reduction of the concentration of the SCFAs. 84 These SCFAs usually obtained from carbohydrate fermentation in the colon are known to be essential components needed for the maintenance of gut homeostasis. However, in a state of gut microbiota dysbiosis, fermentation of these carbohydrates yields a much lower concentration of the SCFAs than they would in healthy states.
Their quantification in stools has been said to possibly function as biomarkers for non-invasive diagnosis for various gut ailments. Thus, Nevertheless, just as Wang and colleagues inferred that microbial metabolites may contribute to CRC development as they observed a reduction in butyrate-producing bacteria in faeces of CRC patients from their study, 85 and some studies also implying that there is an association between bacterial dysbiosis, metabolites and colorectal adenomas, 86 more recent studies found that this association especially, between gut metabolites and colorectal adenoma or CRC is insignificant. 80 This study, however, did not include adenoma patients, and this is essential and should be investigated be explored for future studies.
In the study of Clos-Garcia, however, observable differences These researchers also showed that integrating metabolomics data with that of metagenomics was distinct in the separation of the healthy control group from CRC group as well as adenoma group from CRC group. More researches into these metabolites (sphingomyelins and cholestryl esters) in healthy, adenoma and CRC patients is recommended for the gut microbiota, faecal biomarker study. It is intriguing to note that the bacteria genera found to be differential in the three groups of individuals studied also correlated with the very same metabolite classes majorly observed to be discriminating and differential between sample groups. 40 In furtherance, the development of models that integrate both metabolomics and metagenomics data of sampled individuals is strongly advocated for as such models may improve the sensitivity and specificity of detecting CRC at treatable stages which will in turn, prolong the life span of such individuals. There is no consensus yet on the applicability of the gut-faecal microbiota metabolites as biomarkers for early detection of CRC (Table 2). However, some studies discussed above emphasize that it can be utilized. It is proposed that future studies should further investigate and validate these research findings especially in larger cohorts as the sample sizes utilized by some of the researchers were small. The cost-effectiveness of any screening procedure is important for successful clinical application and affordability is one of the prominent requirements of a good biomarker. Some studies have stated that the application of faecal metagenomics in the diagnosis of CRC is affordable 38 ; however, one notable factor that could contribute to increased diagnostic cost is usually the number of biomarkers employed by some of these studies which could range from 11 to 22. Many developing countries may not be able to integrate this method for CRC screening even though different studies have confirmed its sensitivity over many current non-invasive screening tests. 91 The most discriminatory screening markers should therefore be employed for CRC screening as its benefits will undoubtedly be far-reaching spreading across populations of middle to low-income source.

| CHALLENG E S AND FUTURE PER S PEC TIVE S
There is also the challenge of identifying additional markers with enhanced predictive value and eventually validating them in much larger cohorts involving different nationalities. The prime focus of such exploration would be to discover faecal metagenomics markers that have strong predictive power in diagnosing early-stage CRC, leading to significantly reduced CRC-associated mortality. 38 The data presented in Table 1 show that various studies have tried to validate faecal microbiota signatures in different populations.
However, the development of a few defined universal/regionalbased faecal microbiota panels is essential in order to reduce cost and to overcome the hurdle posed by various factors that could affect the diversity and composition of the gut microbiota. While this is being carried out, it is important to note that some studies have also shown that the Fusobacterium genera alone cannot be used in the distinction between adenoma and carcinoma patients as there is no significant association between this bacterium and the location or stage of the carcinomas. 40,69,71 It is reported to give better predictions on CRC, particularly in the advanced stage but less accurate at TA B L E 2 Summary of research findings on the application of metabolomics as biomarker. the early stage. 73 Future studies can therefore explore the best microbial biomarker panels that integrate Fusobacterium spp. for earlystage diagnosis of CRC.

Major findings
The genera Adlercreutzia should be further investigated in diverse races and ethnic groups to validate its prospect as an efficient diagnostic biomarker of adenoma. Studies on equol production by these bacteria is needed taking the dietary patterns of participants in consideration, and the effect of Adlercreutzia in adenoma patients should be further studied since the equol it produces is associated with a lower risk of CRC. 40 It is important to mention that although many studies have combined the faecal microbial biomarkers identification with FIT in the early detection of CRC, 13,18,70,[91][92][93] one major challenge that has always been encountered is the increase in the number of falsepositive results. The sensitivity obtained by either of these methods is almost always enhanced when they are both combined into a single model for adenoma detection. However, a corresponding improvement in specificity is usually not observed and is rather, reduced. This makes these combination models still not very suitable for early CRC detection as the goal is to reduce the number of primary diagnoses by colonoscopy. Hence, further studies on FIT-faecal microbiota panel combination models are strongly advocated for and even other models that can explore faecal microbial signatures with other non-invasive screening biomarkers for early CRC detection is encouraged.
A graphical presentation summarizing the data and ideas discussed in the manuscript is illustrated in Figure 3.

| CON CLUS ION
The gut microbiota has been shown to provide a reservoir of biomarkers for the early diagnosis of CRC. Early detection of CRC would grossly reduce the high mortality associated with CRC. The best faecal biomarker panel that will improve both the sensitivity and specificity of adenomas detection either applied alone or even in combination with other non-invasive CRC diagnostic methods is achievable and can also be made affordable. This will ensure that more persons are screened and more lives, saved.

CO N FLI C T O F I NTE R E S T
The authors declare that there are no conflicts of interest.