Scientists have known for years that the trillions of microscopic residents in human digestive tracts have a major say in overall health. But until recently, they didn’t know how bacteria were delivering the message.
According to a recent study published in the scientific journal Nature, researchers have identified a molecular pathway that allows gut bacteria to inject their own proteins directly into human cells. This discovery could reshape the understanding of immune responses and chronic conditions like Crohn’s disease.
Researchers previously linked the microbiome to immune disorders, metabolic problems and chronic inflammation. However, most of that evidence was based on correlation.
This study, led by researchers at the German research institute Helmholtz Munich, aimed to change that.
Veronika Young, the study’s first author, explains in a summary of the findings:
“Our goal was to better characterize some of the underlying processes of how gut bacteria affect human biology. By systematically mapping direct protein–protein interactions between bacterial and human cells, we can now suggest molecular mechanisms behind these associations.”
Bacterial syringes can disrupt cell signals
Your gut is full of commensal bacteria that are typically regarded as either friendly or neutral.
But according to the study, those generally harmless gut bacteria can often contain type III secretion systems. These are essentially microscopic syringes that allow the bacteria to inject proteins straight into human cells. Previously, experts believed these systems were exclusive to harmful bacteria like Salmonella.
To understand exactly what bacteria are doing inside human cells, researchers created a massive map showing over a thousand specific interactions between bacterial proteins and human proteins.
They found that these bacterial proteins tend to target the pathways responsible for immune regulation and metabolism. Lab experiments confirmed that the bacteria can actually change key immune signals, specifically affecting pathways involving cytokines.
Cytokines are the signaling molecules that help coordinate the immune system to prevent it from overreacting. But too many cytokines can cause excess inflammation.
The researchers found that people with Crohn’s disease carry more of these specific bacterial genes in their gut.
The gut-inflammation link
This suggests that bacteria injecting proteins directly into human cells could be a driver of chronic inflammation and possibly explain how gut bacteria might cause or worsen inflammatory diseases, a link that scientists had observed but not fully understood until now.
“This fundamentally changes our view of commensal bacteria,” says Pascal Falter-Braun, the study’s corresponding author. “It shows that these non-pathogenic bacteria are not just passive residents but can actively manipulate human cells by injecting their proteins into our cells.”
It’s unclear as to whether these bacterial syringes originated from pathogenic sources or commensal bacteria adapted the injection systems.
However, the researchers say this study can help shift the focus of future studies from correlation to causation, which may result in better treatments for diseases of chronic inflammation that start in the gut.