They discover how bacteria move, putting an end to a 50-year mystery

Researchers at the University of Virginia School of Medicine and their collaborators have solved a decades-long mystery about the ability to move E. coli and other bacteria.

Bacteria propel themselves forward by coiling long, threadlike corkscrew-shaped appendages that act as makeshift propellers. But exactly how they do it has puzzled scientists, because the “helices” are made up of a single protein.

An international team led by Edward H. Egelman of the University of Virginia (UVA), a leader in the field of high-tech cryoelectron microscopy, has cracked the case. The researchers used advanced microscopy, sometimes called cryo-EM, and advanced computer modeling to reveal what no traditional light microscope could see: the bizarre structure of these helices at the level of individual atoms.

Photo Mark AB Kreutzberger et al.

While models of how these filaments could form such regular coiled shapes have existed for 50 years, we have now determined the structure of these filaments in atomic detail.said Egelman, of UVA’s Department of Biochemistry and Molecular Genetics. We can prove these models wrong, and our new understanding will help pave the way for technologies that could be based on these miniature propellers..

Different bacteria have one or more appendages known as flagella. A flagellum is made up of thousands of subunits, but all of them are exactly the same. You might think such a tail would be straight, or at best a bit flexible, but that would leave the bacteria unable to move. That’s because such shapes cannot generate thrust. It takes a rotating corkscrew-shaped propeller to push a bacterium forward. Scientists call the formation of this shape “supercoiling” and now, after more than five decades, they understand how bacteria do it.

Using cryo-EM, Egelman and his team discovered that the protein that makes up the flagellum can exist in 11 different states. It is the precise mixing of these states that causes the corkscrew shape to form.

Salmonella bacteria | public domain photo on Wikimedia Commons

The helix of bacteria is known to be very different from the similar helices used by hardy single-celled organisms called archaea. Archaea are found in some of the most extreme environments on Earth, such as in pools of near-boiling acid, at the bottom of the ocean, and in oil deposits deep under the ground.

Egelman and his colleagues used cryo-EM to examine the flagella of one form of archaea, Saccharolobus islandicus, and found that the protein that makes up its flagellum exists in 10 different states. Although the details were quite different from what the researchers saw in bacteria, the result was the same, with the filaments forming regular corkscrews. They conclude that this is an example of “convergent evolution”, when nature arrives at similar solutions by very different means. This shows that although the helices of bacteria and archaea are similar in shape and function, the organisms evolved those traits independently.

As with birds, bats, and bees, which have independently evolved wings for flight, the evolution of bacteria and archaea have converged on a similar solution for swimming in both.said Egelman, whose previous imaging work earned him induction into the National Academy of Sciences, one of the highest honors a scientist can receive. Given that these biological structures arose on Earth billions of years ago, the 50 years it has taken us to understand them does not seem so long..


Sources

University of Virginia | Mark AB Kreutzberger et al., Convergent evolution in the supercoiling of prokaryotic flagellar filaments, Cell (2022). DOI: 10.1016/j.cell.2022.08.009