Beneficial gut bacteria secrete compounds that rein in a group of immune cells that are involved in inflammatory disorders.

Microbes in the gut help to keep immune responses in check, but how they do this has not been clear. To find out, Richard Blumberg and Dennis Kasper of Harvard Medical School in Boston, Massachusetts, and their team studied a helpful intestinal bacterium, Bacteroides fragilis.

Mice colonized with B. fragilis had fewer 'natural killer T cells' than did mice without the bacterium. But that effect was reduced in mice harbouring B. fragilis that lacked a gene responsible for making fatty compounds called sphingolipids.

Treating these mice with a purified B. fragilis sphingolipid restored normal natural killer T-cell inhibition and protected the mice from chemically induced colitis. Animals exposed to the microbe early in life were more protected than those exposed later.

Cell 156, 123–133 (2014)