how gut bacteria sticks to gut

Researchers have revealed a new molecular mechanism by which bacteria adhere to cellulose fibers in the human gut. Thanks to two different binding modes, they can withstand the shear forces in the body.

Cellulose is a major building block of plant cell walls, consisting of molecules linked together into solid fibers. For humans, cellulose is indigestible, and the majority of gut bacteria lack the enzymes required to break down cellulose.

However, recently genetic material from the cellulose-degrading bacterium R. champanellensis was detected in human gut samples. Bacterial colonization of the intestine is essential for human physiology, and understanding how gut bacteria adhere to cellulose broadens our knowledge of the microbiome and its relationship to human health.

The bacterium under investigation uses an intricate network of scaffold proteins and enzymes on the outer cell wall, referred to as a cellulosome network, to attach to and degrade cellulose fibers. These cellulosome networks are held together by families of interacting proteins.

By using a combination of single-molecule atomic force microscopy, single-molecule fluorescence and molecular dynamics simulations, researchers studied how the complex resists external force.

They were able to show that the complex exhibits a rare behavior called dual binding mode, where the proteins form a complex in two distinct ways. The researchers found that the two binding modes have very different mechanical properties, with one breaking at low forces of around 200 piconewtons and the other exhibiting a much higher stability breaking only at 600 piconewtons of force.

Further analysis showed that the protein complex displays a behavior called a "catch bond," meaning that the protein interaction becomes stronger as force is ramped up.

"We clearly observe the dual binding modes, but can only speculate on their biological significance. We think the bacteria might control the binding mode preference by modifying the proteins. This would allow switching from a low to high adhesion state depending on the environment,"

reference

 Zhaowei Liu et al. High force catch bond mechanism of bacterial adhesion in the human gut, Nature Communications (2020). DOI: 10.1038/s41467-020-18063-x