Many single-celled microorganisms are harmful, but others regulate the immune responses of their animal hosts to guard against infections.

Some such organisms, called protozoa, live in the intestine, but have not been as well studied as their disease-causing counterparts. Miriam Merad at the Icahn School of Medicine at Mount Sinai in New York City and her colleagues identified a previously unknown protozoan, Tritrichomonas musculis, in the intestines of some laboratory mice. When this microbe colonized the guts of other mice, the animals exhibited an inflammatory response that protected against pathogenic Salmonella bacteria. However, the animals also showed increased susceptibility to inflammatory intestinal disease and colon tumours.

Cell 167, 444–456 (2016)