Using 3D images, biologists at Imperial College London, UK, have shown how bacteria have evolved their ‘flagellar’ motors of different powers to optimize their swimming.
Flagellar motors of Wolinella succinogenes, Arcobacter butzleri, and Bdellovibrio bacteriovorus. ‘B,’ ‘M,’ and ‘P’ labels depict Basal disk, Medial ring, and Proximal rings respectively. Image credit: Imperial College London.
Bacteria use molecular motors just tens of nanometers wide to spin a tail (flagellum) that pushes them through their habitat.
Like human-made motors, these nanoscale machines have distinct ‘stator’ and ‘rotor’ components that spin against each other. The structure of these motors determines their power and the bacteria’s swimming ability.
Previously, Imperial College researcher Morgan Beeby and co-authors looked at these motors and discovered a key factor that determined how strongly bacteria could swim.
They found that the more stator structures the bacterial motor possessed, the larger its turning force, and the stronger the bacterium swam.
Despite these differences, DNA sequence analysis shows that the core motors are ancestrally related. This led the team to question how structure and swimming diversity evolved from the same core design.
Now, in new research published in the journal Scientific Reports, Dr. Beeby’s team was able to build a ‘family tree’ of bacterial motors by combining 3D imaging with DNA analysis.
This allowed them to understand what ancestral motors may have looked like, and how they could have evolved into the sophisticated motors seen today.
The scientists found a clear difference between the motors of primitive and sophisticated bacterial species. While many primitive species had around 12 stators, more sophisticated species had around 17 stators. This, together with DNA analysis, suggested that ancient motors may also have only had 12 stators.
“This clear separation between primitive and sophisticated species represents a ‘quantum leap’ in evolution,” the authors said.
“Our study reveals that the increase in motor power capacity is likely the result of existing structures fusing. This forms a structural scaffold to incorporate more stators, which combine to drive rotation with higher force.”
Family tree of the studied bacterial flagellar motors. Image credit: Imperial College London.
To carry out the study, Dr. Beeby and colleagues visualized a number of motors from different species of bacteria (Wolinella succinogenes, Arcobacter butzleri and Bdellovibrio bacteriovorus) using a variant of a method called cryo-election microscopy.
The method involves flash-freezing the motors inside living cells. Once frozen, they can be imaged from all angles to build up a 3D picture of what the motor looks like inside the cell.
The team then built up a ‘family tree’ of the species using DNA sequence analysis, which related their swimming ability and motor properties.
The researchers found that bacteria with 17 or more stators, and their relatives, had extra structures attached to their motors.
They believe that these extra structures fused in sophisticated bacteria to provide a larger scaffold for supporting more stators.
“However, this was likely not a one-time event,” they said. “The extra structures appear to have evolved many times in different species of bacteria, using different building blocks but producing the same functionality.”
“Bacterial motors are complex machines, but with studies like this we can see how they have evolved in distinct steps,” Dr. Beeby added.
“Moreover, the ‘leap’ from 12 stators to 17, while a great innovation, has an aspect of ‘biological inevitability’ in the same way as wings, eyes, or nervous systems in higher animals: the precursors of high torque have evolved multiple times, and one set of them ended up fusing to form the scaffold we describe in our work.”
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Bonnie Chaban et al. 2018. Evolution of higher torque in Campylobacter-type bacterial flagellar motors. Scientific Reports 8, article number: 97; doi: 10.1038/s41598-017-18115-1
