Saturday 4 February 2023

Scientists reveal the structure of bacteria's 'propellers' in near atomic detail

Image:  By Carsten Steger - Own work, CC BY-SA 4.0,


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.


Researchers used cryo-EM and advanced computer modelling to reveal what no traditional light microscope could see: the strange structure of these propellers at the level of individual atoms.


Blueprints for Bacteria's 'Supercoils'


Different bacteria have one or many appendages known as a flagellum, or, in the plural, flagella. A flagellum is made of thousands of subunits, but all these subunits are exactly the same. You might think that 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 can't generate thrust. It takes a rotating, corkscrew-like propeller to push a bacterium forward.


Scientists call the formation of this shape "supercoiling," and now, after more than 50 years, they understand how bacteria do it.


Using cryo-EM, the researchers found that the protein that makes up the flagellum can exist in 11 different states. It is the precise mixture of these states that causes the corkscrew shape to form.



It has been known that the propeller in bacteria is quite different than similar propellers used by hearty one-celled organisms called archaea. Archaea are found in some of the most extreme environments on Earth, such as in nearly boiling pools of acid, the very bottom of the ocean and in petroleum deposits deep in the ground.


The scientists used cryo-EM to examine the flagella of one form of archaea, Saccharolobus islandicus, and found that the protein forming its flagellum exists in 10 different states.


S. solfataricus is a species of thermophilic archaeon. The organism known to grow chemoorganotrophically, in presence of oxygen, on a variety of organic compounds such as sugars, alcohols, amino acids and aromatic compounds like phenol

While the details were quite different than 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 via very different means. This shows that even though bacteria and archaea's propellers are similar in form and function, the organisms evolved those traits independently.

The researchers have published their findings in the scientific journal Cell, titled “Convergent evolution in the supercoiling of prokaryotic flagellar filaments.”

 Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (


  1. Can we call it a shape memory protein, as being displayed in 10 different forms.

  2. I remember learning about cross sections of bacterial flagella and the frictionless motor they run on. A 9+2 arrangement it was called.

  3. I remember in the 80s as a microbiologist student of a 9+2 Cross section arrangement as seen by TEM, along with theories of a near frictionless motor to power its corkscrew behaviour.


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