Many bacteria which can grow in anaerobic conditions end up homing to solid tumors (where there is no more oxygen) and then either directly kill cancer cells fo

Many bacteria which can grow in anaerobic conditions end up homing to solid tumors (where there is no more oxygen) and then either directly kill cancer cells for food, or stimulate a local immune response which can promote clearance of the tumor. W. Bush and F. Fehleisen found back in the 1800's that patients accidentally infected with with Streptococcus pyogenes often had regressions in solid tumors. William Coley's refined this and came with Coley's toxins, which when they worked, worked really well but comes with its own risks: https://en.wikipedia.org/wiki/Coley%27s_toxins https://en.wikipedia.org/wiki/Coley%27s_toxins https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1888599/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1888599/ https://connect.springerpub.com/content/book/978-1-6170-5273-6/chapter/ch01?implicit-login=true https://connect.springerpub.com/content/book/978-1-6170-5273... I think many mycobacterium species also have this effect. Also, bacterial extracts such as the BCG vaccine are used for bladder cancer. More modern strategies try to take advantage of the bacteria's ability to home to the inside of solid tumors in order to deliver a toxic payload to directly kill tumor cells, or molecules which locally wake up the immune system. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7264239/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7264239/ https://www.prnewswire.com/news-releases/synlogic-presents-data-from-synb1891-phase-1-trial-at-american-association-for-cancer-research-aacr-annual-meeting-301266169.html https://www.prnewswire.com/news-releases/synlogic-presents-d... Maybe the clinical results weren't so great though since they seem to have removed it from the website https://www.synlogictx.com/pipeline/ https://www.synlogictx.com/pipeline/. If you count viruses as microbes, then there are people working on viruses that kill tumors as well (oncolytic viruses).