Researchers have solved the 50-year-old mystery of why bacteria and archaea swim using a high-tech microscope.
We have long known they use a little coiled ‘tail’ called a flagellum, but details of how their stringy appendage forms its curled shape to push them forth has eluded our understanding – until now.
In animal cells, flagella work like the tails we’re more familiar with – beating back and forth to propel their bodies forward. However, bacteria cells, the third domain, are single-cellular. archaeaThe flagella, which are corkscrew-shaped and cannot generate thrust from simple side-toside movement, is called the.
These little coils spin like a spinning propeller. Its coils are able to stretch and contract in a similar way to a slinky. This allows microbes to create various waveforms through their motor-driven rotations. You can also alter the direction of rotations.
Both bacteria and archaea flagella both contain the same repeating flagellin subunits. However, the flagellin found within the archaea tail is closer to the one found inside bacteria’s cell protrusion called pili.
Mark Kreutzberger, University of Virginia biophysicist, and his colleagues used this method Cryo-electron tomographyThe molecular structure at the near-atomic level of flagellar filaments in rod-shaped bacteria was examined. Escherichia coliArchaea Saccharolobus islandicus.
They discovered that the protein filaments in bacteria could exist in 11 states and archaea in 10 states. It’s the mixture of these states which causes the whole structure to curl into its coil in both microbes.
The resulting supercoiled structure is so stable it can withstand torsion stresses, retaining its curled shape while being spun – that is, until the flagellum changes spin direction.
In E. coli, straight swimming involves counter-clockwise rotation. However, when bacteria change the spin direction of its tail, the forces exerted on the flagella alter its structure. One or more filaments are loosened from their tight bundling, and the supercoils become semi-coiled, or curled.
This causes the microbe to switch from a straight swimming mode to a tumble. The tail spins clockwise.
The archaea did not experience these direction-induced changes, but changing their environment conditions with salt or acid did change their flagella structure.
Despite their differences in structure and that they evolved independently, nature has shaped both bacteria and archaea’s flagella to essentially have the same form and function – a neat example of convergent evolution.
“As with birds and bats, bees all have evolved wings for flight, but bacteria and archaea have converged to find a similar solution for swimming.” explainsEdward Egelman, University of Virginia biochemist
“Our new understanding will open the door to technologies that can be built upon miniature propellers.”
This research was published in Cell.
