Ending a 50-year mystery, scientists reveal how bacteria can move

Researchers from the University of Virginia School of Medicine and their associates have answered a long-standing question concerning how E. coli and other bacteria may migrate.

By coiling their lengthy, threadlike appendages into corkscrew shapes that serve as improvised propellers, bacteria propulsion themselves ahead. However, because the "propellers" are formed of a single protein, experts are confused as to how exactly they accomplish this.

The case has been solved by an international team led by UVA's Edward H. Egelman, PhD, a pioneer in the high-tech discipline of cryo-electron microscopy (cryo-EM). The unique atomic-level structure of these propellers, which was invisible to a typical light microscope, was revealed by the researchers using cryo-EM and sophisticated computer modeling.

"We have finally determined the structure of these filaments in atomic detail," said Egelman, of UVA's Department of Biochemistry and Molecular Genetics. "Models have existed for 50 years for how these filaments may produce such regular coiled geometries." We can demonstrate that these models were inaccurate, and our new knowledge will make it possible for technologies to be built around these tiny propellers.

Different bacteria have one or more protrusions called flagellums, or flagella in the plural. The thousands of individual parts that make up a flagellum are all identical. You may assume that such a tail would be straight or, at most, slightly flexible, but the bacteria would be unable to move if that were the case. This is because they are unable to produce propulsion. A propeller that rotates and resembles a corkscrew is needed to move a bacteria forward. After more than 50 years, scientists have finally figured out how bacteria produce this shape, which they refer to as "supercoiling."

Egelman and his team discovered the flagellum protein can exist in 11 distinct forms using cryo-EM. The exact combination of these states is what gives rise to the corkscrew shape.

It is well known that bacteria's propeller differs significantly from the robust one-celled creatures known as archaea's propeller. Archaea are organisms that can be found in some of the most hostile settings on Earth, including oil reserves buried deep in the ground and acid pools that are nearly boiling.

When Egelman and colleagues used cryo-EM to look at the flagella of Saccharolobus islandicus, a particular type of archaea, they discovered that the protein that makes up its flagellum existed in ten distinct states. The filaments formed typical corkscrews, however the specifics were significantly different from what the researchers observed in bacteria. They come to the conclusion that this is an illustration of "convergent evolution," which occurs when nature finds solutions through quite dissimilar approaches. This demonstrates that although the propellers of bacteria and archaea are identical in structure and function, the features were separately evolved by the two groups of organisms.

Egelman, whose prior imaging work saw him inducted into the National Academy of Sciences, one of the highest honors a scientist can receive, said that just as birds, bats, and bees independently evolved wings for flying, the evolution of bacteria and archaea "converged on a similar solution for swimming in both." "The 50 years it has taken to grasp these biological structures may not seem like a long time given that they first appeared on Earth billions of years ago."

The scientific journal Cell has published the findings of the researchers. Priyanka Biswas, Amanda L. Sebastian, Cheryl Ewing, Weili Zheng, Frédéric Poly, Gad Frankel, B.F. Luisi, Chris Calladine, Mart Krupovic, Birgit E. Scharf, and Egelman made up the team. Ravi R. Sonani, Junfeng Liu, Sharanya Chatterjee, Fengbin Wang, Amanda L. Sebastian, and Egelman also participated.

The U.S. Navy Work Unit Program 6000.RAD1.DA3.A0308, the Robert R. Wagner Fellowship, and the National Institutes of Health's grants GM122150 and T32 GM080186 all provided funding for this project. The research team's work does not represent the Department of the Navy, the Department of Defense, or the American government's official policy or position.

University of Virginia Health System