What are the most helpful microorganisms in a mammal's body?

I would say that the microorganisms in your gut are the most helpful, but then I would say that since I work on them. But in terms of specifics then there are two main species that we are most interested in. They are very helpful in producing butyrate, which is protective against colon cancer, and are called Roseburia and Faecalibacterium prausnitzii. Faecalibacterium prausnitzii has also been shown to not be present in people who have Crohn's disease. So there's currently a lot of work going on in various countries around the world to try and see if you can re-introduce that specific bacterium back into patients who have Crohn's disease and whether you can avoid or alleviate the symptoms.