Colorectal cancer (CRC) is the second most common identified malignancy worldwide and thus represents an important health and socioeconomical burden. Recent data based on metagenomics and experimental models suggest a strong contribution of the gut microbiome in modulating CRC development. Various studies have systemically showed a significant shift in microbial composition comparing the gut microbiome of CRC patients with healthy individuals, a phenomenon commonly referred to as dysbiosis. Particularly, the presence of certain types of bacterial populations such as Bacteroides fragilis, Escherichia coli, and Fusobacterium nucleatum have been associated with an increased risk for the development of CRC, while other bacteria including Lachnospiraceae species seem to have an antitumor effect in the colon by producing metabolites such as short-chain fatty acid butyrate promoting apoptosis of colonic cancer cells.
Radiotherapy plays an adjuvant role in the treatment of CRC. However, the usage of radiotherapy has been shown to cause mucositis and radiation-induced ulceration which drive substantial changes in the gut microbiome leading to intestinal dysbiosis in CRC patients. This radiation‐induced toxicity and its association with microbial dynamics, forms a medical problem that urgently needs addressing in order to have an effective intervention.