Author contributions: Cipe G and Idiz UO contributed equally to this work; Cipe G, Idiz UO, Firat D and Bektasoglu H designed the research; Idiz UO and Firat D performed the research; Idiz UO, Firat D and Bektasoglu H wrote the paper. Conflict-of-interest statement: The authors declare that they have no conflict of interest. Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/ licenses/by-nc/4.0/ Abstract The human gastrointestinal tract hosts a complex and vast microbial community with up to 1011-1012 microorganisms colonizing the colon. The gut microbiota has a serious effect on homeostasis and pathogenesis through a number of mechanisms. In recent years, the relationship between the intestinal microbiota and sporadic colorectal cancer has attracted much scientific interest. Mechanisms underlying colonic carcinogenesis include the conversion of procarcinogenic diet-related factors to carcinogens and the stimulation of procarcino- genic signaling pathways in luminal epithelial cells. Understanding each of these mechanisms will facilitate future studies, leading to the development of novel strategies for the diagnosis, treatment, and prevention of colorectal cancer. In this review, we discuss the relationship between colorectal cancer and the intestinal microbiota. Key words: Sporadic; Colorectal; Cancer; Intestinal; Microbiota © The Author(s) 2015. Published by Baishideng Publishing Group Inc. All rights reserved. Cipe G, Idiz UO, Firat D, Bektasoglu H. Relationship between intestinal microbiota and colorectal cancer. World J Gastrointest Oncol 2015; 7(10): 233-240 Available from: URL: http://www. wjgnet.com/1948-5204/full/v7/i10/233.htm DOI: http://dx.doi. org/10.4251/wjgo.v7.i10.233 INTRODUCTION Colorectal cancer is the third commonest cancer type Core tip: Microbiota’s role in providing intestinal homeo- stasis is not as an audience, but it is active. Both the composition of microbiota and its metabolic activity impact the sensitivity of the host and can cause many pathologies including colorectal cancer. WJGO| www.wjgnet.com 233 October 15, 2015| Volume 7| Issue 10| Cipe G et al . Intestinal microbiota and colorectal cancer associations worldwide and causes 600000 deaths every year. Because colorectal cancer patients are frequently asymptomatic in the early phase of the disease, diagnosis at this stage presents a significant clinical challenge. Detection of early stage cancers (stages 1-2) allows curative surgery with a 5-year survival rate of 80%. However, survival rates decrease to approximately 10% for metastatic and late stage tumors. Although there are currently methods for the early diagnosis methods, including computed tomography, colonoscopy, and blood tests, it is expected that evaluation of the intestinal microbiota will prove to be a valuable method allowing earlier diagnosis of colorectal cancer. In humans, a relationship between cancer and microorganisms has been demonstrated in a number of organs, with the most well-known example being the relationship between Helicobacter pylori and gastric cancer and mucosa-associated lymphoid tissue lymphoma. In adults, while the bacterial population in the stomach and small intestine is smaller (103-104 CFU/g contents), increased concentrations of microorganisms are found in the colon (1011-1012 CFU/g contents) compared with the upper gastrointestinal tract. The majority of these microorganisms exist in a favorable symbiotic relationship with humans[3,4]. The intestinal microbiota develops specific to individual variation and environmental conditions beginning at birth. Recently, etiology of colorectal cancer has been shown to be related to genetic mutations, diet, infla- mmatory processes, lifestyle, and the gut micro- biota, with up to 95% of colorectal cancer thought to sporadically develop in individuals with no genetic predisposition. The colonic microbiota is thought to contribute to the development of colorectal cancer by controlling the epithelial cell proliferation and differentiation, synthesizing essential nutrients and bioactive products, preventing the reproduction of pathogenic organisms, and stimulating the immune system. In this review, studies investigating the role of the intestinal microbiota in the development of colorectal cancer development are discussed.