Immunity 31, 677–689 (2009) ; Cell doi:10.1016/j.cell.2009.09.033 (2009)

Hundreds of beneficial bacterial species live in the human gut and help balance the immune system, but which species provide the benefit and how are not well known. Two research groups have pinpointed only a few select species that seem to do much of the work in boosting immune responses.

The groups measured levels of intestinal T cells in mice after challenging them with various microbes. They found that the most potent immune stimulators were segmented filamentous bacteria. Valérie Gaboriau-Routhiau of INSERM in Paris and her colleagues found that, on their own, these bacteria triggered the same broad T-cell response normally caused by the entire mouse intestinal flora.

Dan Littman of New York University School of Medicine, Kenya Honda at Osaka University in Japan and their colleagues showed that the bacteria induced accumulation of helper T cells, which produce cytokines that mediate a variety of immune responses.