Tumor-Targeting Bacteria: As Vectors, Immunotherapeutic Agents And Tumor-Targeting Probes For Cancer Detection And Therapy

Cancer is the world's second leading cause of death in humans. Conventional anticancer therapies are often associated with lack of tumor specificity, failure to detect small metastases, increased resistance of tumors to anticancer drugs, and unintended adverse effects. Numerous alternative and better strategies in cancer treatment have been developed to overcome the negative effects of traditional cancer therapies. More than a century ago, William Coley, the father of cancer immounotherapy, laid the groundwork for bacterial anticancer therapy. Bacterial immunotherapy has been emerging as a potential anticancer therapy. Moreover, certain obligate and facultative anaerobic bacterial species are exploited as vectors for gene delivery to treat cancer. These genes encode for anticancer agents, cytokines, cytotoxic peptides, anti-angiogenic proteins, therapeutic molecules and prodrug-converting enzymes. Genetically engineered bacterial strains of Salmonella, Bifidobacterium, Clostridium and Listeria are widely used to deliver genes in anticancer therapy since they can selectively accumulate in solid tumors with a hypoxic/necrotic core in vivo, providing appealing delivery systems to target therapeutic agents and immunomodulatory molecules to the site of tumor. Certain genetically modified bacterial species such as Bifidobacterium longum and Bacillus licheniformis have been effectively used for the enzyme/prodrug therapy for cancer. Furthermore, certain anaerobic bacteria are emerging as potential tumor markers due to the increased mobility and the selectivity in germinating and multiplying in hypoxic/anoxic environments. Many of these novel developments have been studied extensively in different experimental models of cancer and certain clinical trials are ongoing for some treatment modalities. Although favourable results have shown so far, further studies and technological innovations are required to ensure the efficacy of bacterial anticancer therapy.