Bacteria that thrive in the guts of humans and other mammals make a molecule that goads crucial immune cells into action, thus helping to repel invasive viruses.
The mammalian gut is occupied by trillions of harmless bacteria, including the abundant species Bacteroides fragilis. To investigate the microbiome–host relationship, Dennis Kasper at Harvard Medical School in Boston, Massachusetts, and his colleagues analysed how a molecule in B. fragilis’s outer membrane affects the immune system of mice. The team looked in particular at the rodents’ dendritic cells, which act as the scouts of the immune system.
The researchers found that when dendritic cells were exposed to the membrane molecule, they secreted a powerful signalling chemical called interferon-β. That chemical in turn switched on a battery of genes that affect the immune response. Dendritic cells that were combined with the bacterial molecule in a laboratory dish largely fended off infection by influenza A virus, but almost half of the cells in a control group became infected.
Many relatives of B. fragilis have membrane molecules that stir a similar response, the authors say.