Chemotherapy is used frequently for treating tumors. For many kinds of tumors, chemotherapy is an effective way of treatment. Besides the desired effects on neoplastic cells, the cytotoxic effects often lead to undesirable effects in other cells. Particularly the epithelial tissue of the gastrointestinal tract is affected, mucositis and diarrhea occur. By loss of water and electrolytes, diarrhea often make delay in chemotherapy necessary. Besides mucositis, a chemotherapy also causes changes in gut microbiota as animal models proof. It is well known, than probiotics shorten diarrhea. So far, the use of probiotics in chemotherapy patients was only investigated in one single study. Severity of diarrhea was reduced compared to placebo. Escherichia coli Nissle 1917 (EcN) is a well investigated probiotic. In cell culture, the supernatant of EcN reduced the noxious effect of 5-Fluoruracil concerning cell toxicity and disruption of barrier function.