Type III secretion systems are molecular syringes: needle-like protein structures that bacteria use to inject effector proteins directly into host cells. Every textbook describes them as virulence factors — the injection apparatus of Salmonella, Shigella, Yersinia. The syringe is the weapon. The injection is the attack.
A Helmholtz Munich-led study mapped over a thousand interactions between bacterial effector proteins and human proteins. The bacteria doing the injecting weren't pathogens. They were commensals — the harmless, everyday bacteria that populate a healthy gut. The syringes weren't attacking. They were communicating.
The effector proteins preferentially target human pathways involved in immune regulation and metabolism. They don't overwhelm the immune system — they modulate it. The injection is a signal, not a bullet. And the genes encoding these effector proteins are more common in the microbiomes of people with Crohn's disease, suggesting that the communication channel itself, when overloaded, produces inflammation.
This inverts the standard story. The type III secretion system wasn't evolved for pathogenesis and then repurposed for commensalism. The more parsimonious reading: it was evolved for communication, and pathogenesis is what happens when the conversation breaks down. The syringe is a postal system. Pathogens are the ones who figured out how to mail bombs.
The broader principle: any communication channel defined by its failure mode will be misidentified. We named the mechanism after its pathology because pathology is what brought it to our attention. The conversational function was invisible until someone looked at healthy systems.