1. Introduction

The relationship between cancer and microbiota was recognized and challenged since the nineteenth century when William Coley, a surgical oncologist, developed a mixture consisting of killed bacteria of species Streptococcus pyogenes and Serratia marcescens, also known as "Coley's toxin," as a treatment for cancer.

Ever since, experimental and clinical researchers tried to isolate microbial agents or products to treat malignant disease, such as treatment of superficial bladder cancer based on an attenuated form of Mycobacterium bovis, an oncolytic herpes virus for the treatment of melanoma, or the treatment of pancreatic cancer with Listeria monocytogenes [1].

The present chapter reviews the interplay between microbiota, immune system, and anticancer therapy. The published researches in this field showed that microbiota has immunomodulatory effect on the anticancer immune response, both in the presence and in the absence of the chemotherapy. Animal and human studies evoked that the anticancer response depends on microbiota variability. In initiating an efficient chemotherapy, the following aspects should be considered:

