How the immune system differentiates between pathogenic and harmless gut bacteria is reported online this week in Nature Immunology.

Gabriel Nunez and colleagues looked at infection by pathogenic bacteria and compared them to non-pathogenic gut bacteria. Intestinal phagocytes-immune cells that ingest harmful bacteria or dying cells-were completely unresponsive to all bacteria tested, with one exception: only pathogenic bacteria could stimulate high amounts of the inflammatory cytokine IL-1. The authors found that intestinal phagocytes used an intracellular protein complex to distinguish between pathogenic and harmless bacteria.