Scientists solve the 50-year-old mystery of how bacteria move

It all has to do with the flagella.
Loukia Papadopoulos
Bacteria use proteins to move.
Bacteria use proteins to move.


When it comes to bacteria, you may think we have it all figured out but some mysteries remain for decades like how these beings move. Now, an international team led by UVA’s Edward H. Egelman, PhD, a leader in the field of high-tech cryo-electron microscopy (cryo-EM), has figured that out, according to a press release by the institution published on Monday.

The findings have been published in the journal Cell.

Pushing themselves forward

“Bacteria push themselves forward by coiling long, threadlike appendages into corkscrew shapes that act as makeshift propellers. But how exactly they do this has baffled scientists, because the “propellers” are made of a single protein,” stated the scientists in their press release.

So, the researchers used cryo-EM and advanced computer modeling to reveal what no traditional light microscope could: how these bacteria move at the level of individual atoms. This process would prove a game changer unleashing new never-before-seen findings about bacteria and their movements.

“While models have existed for 50 years for how these filaments might form such regular coiled shapes, we have now determined the structure of these filaments in atomic detail,” said Egelman, of UVA’s Department of Biochemistry and Molecular Genetics.

Scientists solve the 50-year-old mystery of how bacteria move
How do bacteria move?

“We can show that these models were wrong, and our new understanding will help pave the way for technologies that could be based upon such miniature propellers.”

Bacteria have one or many appendages known as a flagellum (in the plural flagella) to push them forward, shaped like rotating, corkscrew-like propellers. Scientists refer to the formation of their shape as “supercoiling” but have been long perplexed by how bacteria do it since the flagellum consists only of proteins.

Existing in several different states

The researchers, however, now uncovered that these proteins can exist in 11 different states that can lead to several interesting forms. “It is the precise mixture of these states that causes the corkscrew shape to form,” write the researchers.

The researchers further compared these flagella to similar propellers used by hearty one-celled organisms called archaea using cryo-EM to examine the flagella of one form of archaea called Saccharolobus islandicus.

They found that the protein forming this flagellum exists in 10 different states and that the filaments formed regular corkscrews.This is a clear example of “convergent evolution” (when nature arrives at similar solutions via very different means) and indicates that although the flagella and archaea are quite dissimilar the independently evolved to function in a surprisingly similar fashion.

“As with birds, bats and bees, which have all independently evolved wings for flying, the evolution of bacteria and archaea has converged on a similar solution for swimming in both,” said Egelman, whose prior imaging work saw him inducted into the National Academy of Sciences, one of the highest honors a scientist can receive.

“Since these biological structures emerged on Earth billions of years ago, the 50 years that it has taken to understand them may not seem that long.” Now, if they could only figure out how bacteria use light to breathe electricity they would have solved yet another mystery.


The supercoiling of bacterial and archaeal flagellar filaments is required for motility. Archaeal flagellar filaments have no homology to their bacterial counterparts and are instead homologs of bacterial type IV pili. How these prokaryotic flagellar filaments, each composed of thousands of copies of identical subunits, can form stable supercoils under torsional stress is a fascinating puzzle for which structural insights have been elusive. Advances in cryoelectron microscopy (cryo-EM) make it now possible to directly visualize the basis for supercoiling, and here, we show the atomic structures of supercoiled bacterial and archaeal flagellar filaments. For the bacterial flagellar filament, we identify 11 distinct protofilament conformations with three broad classes of inter-protomer interface. For the archaeal flagellar filament, 10 protofilaments form a supercoil geometry supported by 10 distinct conformations, with one interprotomer discontinuity creating a seam inside of the curve. Our results suggest that convergent evolution has yielded stable superhelical geometries that enable microbial locomotion.

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