How does our gut recognize which bacteria are good and which are bad? That’s a pretty tough question, and it depends on the receptors. Toll-like receptors recognize what are called molecular models of bacteria. Pathogenic bacteria have separate models, so the cells determine that such a bacterium is various from the one that should inhabit us. They are resistant to "friends" (good bacteria) and live with them in symbiosis. The presence of the "enemy", in turn, causes anxiety and activates the immune system.