Microbiota: A Double-Edged Sword in Cancer Therapy

The relationship between the host and its microbiota can be symbiotic or pathogenic depending upon the body’s immunity standards. Experiments have demonstrated that germ-free or antibiotics treated mice showed increased colorectal and pancreatic cancer risk. This shed light on the central role of microbiota in inflammation and immune system regulation of the host; both get impaired during cancer development. Microbial dysbiosis depending on the host’s genotype, antibiotics exposure, diet alteration, and cancer therapy can either promote or suppress the diseased state. Thus, the host and its microbiota association is being dissected thoroughly and there exists a possibility in using microbiota as a target or a weapon to kill tumors. Cancer treatment remains a challenge to date due to the heterogeneity of solid tumors concerning the genetic make-up, histopathological features, and clinical behaviors. Bacteria can be used as gene/drug delivery vehicles (weapon) to target tumors. Besides, intratumoral bacteria can also be targeted to indirectly inhibit cancer growth. This is because microbial signals induce anti-tumor immunity via CD8+ T cell activation, prime myeloid cells for TNF production in response to CpG oligonucleotides, produce ROS in response to platinum salts (oxaliplatin), and regulate the efficacy of anti-CTLA-4 and anti-PD-L1 therapy. Therefore, in combination with these conventional cancer therapies (chemotherapy and immunotherapy), consideration of microbial consortium of the patient is a novel approach and has helped increase the efficacy of anti-tumor therapies. Here, in this review, the aim is to elucidate the role played by microbiota in cancer development and how it can be manipulated for tumor targeting, and enhancing cancer therapy. Also discussed are other clinical regimens like probiotics and fecal microbiota transplantation wherein the role of microbiota is central.