Bacteria under transmission electron microscope flagella
By using a high-tech microscope to observe rapidly frozen proteins, researchers have just solved a mystery from 50 years ago, about how bacteria and their ancient enemies, archaea, swim.
We have long known that they use a small curly "tail" called flagella, but until now it is unclear how their slender appendages form curly shapes to push them forward.
In animal cells, flagella works like the tail we are more familiar with - swinging back and forth to push their bodies forward. However, cells belonging to bacteria, as well as the third field of life, single-cell archaeal , have spiral flagella, and cannot generate thrust through simple left and right movements.
Instead, these small coils rotate like propellers. Its coils seem to be able to stretch and contract in some way, similar to a corset, allowing microorganisms to create different waveforms through motor-driven rotations. Rotation can also change direction.
Both bacteria and archaeal flagella consist of the same flagellin repeat subunit . However, the type of flagellin found in archaeal tails is more similar to the type of flagellin found in another cellular protrusion found in a bacteria called pilates.
flagellar structure differences between bacteria and archaea
Biophysicist Mark Kreutzberger and colleagues used cryoelectron tomography to examine the molecular structure of rod-shaped bacteria E. coli and archaeological Saccharolobus islandicus flagellar filaments at a near-atom level.
They found that protein filaments can exist in 11 different states in bacteria and in archaea, and in 10 different states. Despite the differences in protein structure, it mixes these states, causing the overall structure of both microorganisms to be curled into curly shapes.
The resulting superhelical structure is very stable and can withstand torsional stress, maintaining its spiral shape when rotated—that is, until the flagella changes its rotation direction.
In E. coli, straight-line swimming involves counterclockwise rotation. But when the bacteria change the rotation of the tail, the force exerted on the flagella changes its structure, twisting one or more filaments out of the tight bundle and loosening the superspiral into a semi-spiral or coiled shape.
This changes the microorganism's straight swimming mode to the tail now rotates clockwise.
tumbling (curling) flagella pattern (shown in blue) and straight-swimming (normal) flagella pattern (shown in purple)
Although changing their environmental conditions by adding salt or acid does change the structure of their flagella, these directionally induced changes were not observed in Archaeophageal .
Despite their structural differences and they evolve independently, nature has shaped the flagella of bacteria and archaeobacteria into essentially the same form and function - a good example of the convergent evolution of .
"Just like birds, bats, and bees all independently evolved into flying wings, the evolution of bacteria and archaea is focused on a similar swimming solution," explains Edward Egelman, a biochemist at the University of Virginia.
"Our new understanding will help pave the way for technology based on this micropropeller."
