Microbial dysbiosis and colon carcinogenesis: could colon cancer be considered a bacteria-related disease?

Abstract

Colorectal cancer (CRC) is posing an increasingly important burden on the health care system, with western countries seeing a growing incidence of the disease. Except for germline DNA mutations which have been attributed to less than 5% of patients, little is known about the main causes of CRC. However, environment factors such as food, lifestyle and medication are now suspected to have a major influence on inducing cancers. Today, exhaustive quantitative and qualitative evaluation of all environmental factors is not possible. Various environment-induced diseases have been characterized based on colon microflora, also called microbiota, analyses. Growing data have shown specific changes in microflora (i.e. dysbiosis) in the stools of patients with colon cancer or those adherent to the colonic mucosa. Thus, it appears that microbiota may be considered a platform offering host and environment interactions for studying CRCs. The hypothesis that colon cancer might be a bacteria-related disease is suggested and perspectives are discussed.

Keywords: bacteria; bacteria–host interaction; colon cancer; environment; genetic.

Figure 1.

Composition of bacteria in the stools of patients with colon cancer. Fresh samples were collected prior to the colonoscopy, DNA was extracted and submitted to pyrosequencing analyses of the 16S rRNA V3–V4 region and comparisons of bacteria sequences were performed between individuals with normal colonoscopy and patients with colon cancer on the basis of bacterial species abundance. This region belongs to the phylogenetic core and differentiates patients with cancer from healthy individuals. Principal component analysis, based on the 16S rRNA gene sequence abundance of 10 discriminate phylogenetic core species, was carried out with six normal individuals (red points) and six patients with cancer (black points) with two replicates. Two first components (PC1 and PC2) were plotted and represented 57.95 % of the whole inertia. Individuals (represented by their sample id) were clustered and centre of gravity computed for each class. This is the first comparison of stool composition in healthy individuals and patients with colon cancer and shows significant dysbiosis (figure is due to Julien Tap).

Figure 2.

Fusobacterium in colorectal tumours [Kostic et al. 2012]. (a) Fluorescence in situ hybridization (FISH) using an Oregon-Green 488-conjugated ‘universal bacterial’ 16S rDNA-directed oligonucleotide probe (EUB338, green) (top left), and Cy3-conjugated Fusobacterium (FUSO, red) (top right and bottom centre) 16S rDNA-direct oligonucleotide probe demonstrates the presence of bacteria and Fusobacterium in the colonic mucosa of colorectal tumour samples. Representative images are shown with a 10 μm scale bar in the lower corner of each panel; white arrowheads mark bacteria. Epithelial cell nuclei were stained with 4’,6-diamidino-2-phenylindole. (b) Shows whether Fusobacterium was enriched in tumour versus normal pairs. Each dot represents data from either a tumour or a normal sample from nine tumour/normal paired cases. The mean, standard error of the mean and p values (calculated by a Wilcoxon matched-pair signed rank test) are shown.

Figure 3.

Fusobacterium invading colon cancer Caco-2 cells, in vitro [Castellarin et al. 2012]. Representative differentially stained immunofluorescence image showing strain CC53 invading Caco-2 cells. (a) The differential staining method allows for delineation between bacteria that have penetrated the host cells (labelled for actin in green) to reside within them (orange, also indicated with orange arrows), and bacteria present on the outside of the cell (purple). CC53 shows a very long, fine, thread-like cell morphology. (b) Detail of CC53 invasion. Top right: a representative CC53 cell in the process of invading the Caco-2 host cell [image is differentially stained as for (a)]. The long, thread-like cells appear to penetrate the host cell pole first. Orange arrow: the CC53 cell that is internalized. White arrow: the external portion of the same bacterial cell that has looped around on itself, demonstrating apparent flexibility. Purple arrow: a single CC53 cell that has not invaded the host cells, for contrast. The scale bar is 15 μm. In the immunofluorescence micrograph, CC53 shows a very long flexible cell morphology. Green: actin (Caco-2 cells); orange: invasive and internalized bacteria; purple: bacteria external to the cell.

Figure 4.

Colorectal carcinogenesis from normal to invasive cancer. The slides on the top are from colonoscopy after cleaning preparation. The genes involved in each step are also indicated. High magnification shows aberrant crypt foci is needed to detect precancerous lesions in macroscopic normal mucosa. APC, adenomatous polyposis coli; K-ras, Kirsten rat sarcoma; MMR, mismatch repair; MSI, microsatellite instability.