Bugs as drugs: neglected but a promising future therapeutic strategy in cancer.

Effective cancer treatment is an urgent need due to the rising incidence of cancer. One of the most promising future strategies in cancer treatment is using microorganisms as cancer indicators, prophylactic agents, immune activators, vaccines or vectors in antitumor therapy. The success of bacteria-mediated chemotherapy will be dependent on the balance of therapeutic benefit and the control of bacterial infection in the body. Additionally, protozoans and viruses have the potential to be used in cancer therapy. This review summarizes how these microorganisms interact with tumor microenvironments and the challenges of a 'bugs as drugs' approach in cancer therapy. Several standpoints are discussed, such as bacteria as vectors for gene therapy that shuttle therapeutic compounds into tumor tissues, their intrinsic antitumor activities and their combination with chemotherapy or radiotherapy. Bug-based cancer therapy is a two-edged sword and we need to find the opportunities by overcoming the challenges.