The protein FimH sits at the tip of projections on the surface of many enterobacterial strains and grips human cells. It has been thought that bacteria lose that grip when flushed out by body fluids such as saliva. But Thomas et al. now show that the stress of getting caught mid-stream can actually make E. coli clasp on tight, and it's FimH that does the grasping.

In the 28 June issue of Cell, the authors revisited a decades-old observation that shaking up bacteria with red blood cells in a flask causes the red blood cells to clump. If the flask is then left to sit, Thomas et al. discovered that certain strains of E. coli fall off the red blood cells, causing them to disaggregate. The reason for this counter-intuitive behavior has to do with the mechanics of FimH.

The authors delved into the mechanics of FimH by applying a new computational method to the previously solved structure of the protein. That method, called steered molecular dynamics (SMD), stretches a known protein structure under an external force. The authors performed SMD simulations designed to mimic tension mediated by the sheer stress of fluids. As in this image, sheer force stretches the segment (in pink) that connects FimH to bacterial fimbrial projections. The authors describe the mechanism as a “finger-trap” in which the harder you pull, the more your finger gets stuck in a trap. Sheer stress could shift the binding site (in green) from a “low-affinity” to a “high-affinity” conformation, or expose an additional binding site, speculate the researchers. The authors note that with this mechanism, bacteria are well adapted to not only clamp down under stress, but to pick up and move under their own power when the coast is clear.