Here is one of many examples of different forms of targeted cancer therapies under development. Most use nanoparticles, immune cells, or viruses as the agent that selectively transports a cell-killing mechanism to the cancer cells, but bacteria are also a viable possibility:
[Researchers] have developed a therapy for pancreatic cancer that uses Listeria bacteria to selectively infect tumor cells and deliver radioisotopes into them. The experimental treatment dramatically decreased the number of metastases (cancers that have spread to other parts of the body) in a mouse model of highly aggressive pancreatic cancer without harming healthy tissue.
Several years ago, scientists observed that an attenuated (weakened) form of Listeria monocytogenes can infect cancer cells, but not normal cells. [The] tumor microenvironment suppresses the body's immune response, allowing Listeria to survive inside the tumors. By contrast, the weakened bacteria are rapidly eliminated in normal tissues. Scientists later showed that Listeria could be harnessed to carry an anti-cancer drug to tumor cells in laboratory cultures, but this concept was never tested in an animal model. These findings prompted [researchers to couple] a radioactive isotope called rhenium to the weakened Listeria bacteria. "We chose rhenium because it emits beta particles, which are very effective in treating cancer. Also, rhenium has a half-life of 17 hours, so it is cleared from the body relatively quickly, minimizing damage to healthy tissue."
Mice with metastatic pancreatic cancer were given intra-abdominal injections of the radioactive Listeria once a day for seven days, followed by a seven-day "rest" period and four additional daily injections of the radioactive bacteria. After 21 days, the scientists counted the number of metastases in the mice. The treatment had reduced the metastases by 90 percent compared with untreated controls. In addition, the radioactive Listeria had concentrated in metastases and to a lesser extent in primary tumors but not in healthy tissues, and the treated mice did not appear to suffer any ill effects.