Scientists have recruited modified bacteria to help fight cancer, which successfully infiltrated tumors and activated the immune system to kill malignant cells, a new study reports. Tumors size decreased below detectable limits in 11 out of 20 mice that received injections of a strain of bacteria designed to be innocuous, yet able to effectively suppress the growth of cancerous masses. Despite the fact that Salmonella strains have been harnessed to deliver different types of therapeutic agents, these strategies often require multiple injections of microbes, and relapse is common. In search of a better method, Jin Hai Zheng and colleagues used attenuated Salmonella typhimurium bacteria as "Trojan horses," which infiltrated the low-oxygen environments found within tumors and secreted an immune response-triggering signal - from a protein named FlaB, involved in the locomotion of the marine microbe Vibrio vulnificus -- that stimulated the cancer-eliminating activities of protective macrophages. The FlaB-expressing bacteria was proven to be nontoxic, and importantly, didn't invade non-cancerous tissue in rodents. After three days post-administration, the numbers of bacteria inside tumors were 10,000-fold greater than those found in vital organs. What's more, the combination of Salmonella and FlaB synergistically shrank tumors, prolonged survival and also prevented metastasis in a mouse model of human colon cancer. While mice receiving non-FlaB producing microorganisms displayed some reductions in cancer burden, their tumor masses tended to regrow. The authors speculate that its good safety profile makes the engineered bacteria a promising potential anticancer strategy.