How Bacteria Move Reveled

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

Bacteria push themselves forward by coiling long, thread-like accessory parts into the shape of a bottle opener, like temporary propellers. But how they do this puzzles scientists because the propeller consists of a single protein.

An international team led by Dr. Edward H. Egelman of the University of Virginia has addressed this issue. Dr. Edward H. Egelman is a leader in the field of high-tech cryo-electron microscopy (cryo-EM). The researchers used cryo-electron microscopy and advanced computer models to reveal what traditional light microscopy could not see: the strange structure of these propellers at the level of individual atoms.

Egelman of the Department of Biochemistry and Molecular Genetics at the University of Virginia said: "Although models of how these fibers form regular curly shapes have been available for 50 years, we have now determined the atomic detail structure of these fibers." "We can show that these models are wrong and our new understanding will help pave the way for technologies based on this miniature propeller."

Different bacteria have one or more accessory components called flagella, which consist of thousands of subunits, but all of them are identical. You might think that the tail should be straight, or at best a bit flexible, but that would keep the bacteria from moving. This is because such a shape cannot generate thrust. It requires a rotating spiral propeller to push the bacteria forward. Scientists call this shape a superhelix, and now, more than 50 years later, they understand how bacteria do this.

Using cryo-electron microscopy, Egelman and his team found that proteins that make up flagella can exist in 11 different states. It is the precise mixing of these states that leads to the formation of spirals.

It has been known that propellers in bacteria are distinct from similar propellers used by healthy unicellular organisms called Archaea. Archaea are found in some of the most extreme environments on Earth, such as near boiling acid pools, at the bottom of the ocean, and in petroleum deposits deep underground.

Egelman and his colleagues used cryo-electron microscopy to examine the flagella of an archaea, Saccharolobus islandicus, and found that the proteins that form the flagella exist in 10 different states. Although the details were very different from what researchers saw in bacteria, the results were the same, and the fibers formed regular bottle openers. They concluded that this is an example of "convergent evolution" — when nature achieves similar solutions through very different ways. This suggests that although the propellers of bacteria and Archaea are similar in form and function, these organisms have independently evolved these features.

"Like birds, bats, and bees, they have independently evolved flying wings, and the evolution of bacteria and archaea has converged to a similar swimming method," Egelman said. His previous imaging work led him to the National Academy of Sciences, one of the highest honors scientists can receive. "Since these biological structures emerged on Earth billions of years ago, it has taken us 50 years to understand them and may not look long."