New study shows how bacteria evolved powered-up propellers

A shoal of swimming Campylobacter jejuni bacteria, with one in close-up showing its supercharged flagellar motor.

Dr Morgan Beeby’s lab has discovered how some flagella - tiny ‘tail’ propellers used by bacteria for swimming - evolved to be more powerful.

Flagella are helical propellers rotated by molecular machines called flagellar motors. They enable bacteria to swim to sites of infection to cause diseases ranging from food poisoning and ulcers to syphilis and Lyme disease. How flagellar motors work and how they evolved, however, remains incompletely understood. Flagellar motors are embedded in the surfaces of bacteria, making studying them difficult: conventional techniques to study such nanomachines involves removing them from the cell, but like car motors, flagellar motors are not easily extricated, and tend to fall apart when extracted from the bacterium.

Now, Dr Morgan Beeby’s team has developed a new approach to visualise flagellar motors while still inside bacteria, providing high resolution 3-D images of intact motors. The images are to such high resolution that the team could discern the molecular-scale architecture of the motor. The work is published in Nature Microbiology.

Nature’s engineering in action

"We aim to do at the scale of bacteria what Darwin did at the scale of birds by building a ‘portrait gallery’ of different flagellar motors".  Dr Morgan Beeby Associate Professor in Structural Biology

Flagella are elegant molecular-scale remedies to the same problems solved by the wings of a bird or fins of a fish.

By studying flagella, researchers are uncovering principles of how molecular machines work, and of how evolution works at the molecular-scale - a nanoscale echo of Darwin’s work on the beaks of finches in the Galápagos islands.

Dr Beeby said: 'We aim to do at the scale of bacteria what Darwin did at the scale of birds by building a ‘portrait gallery’ of different flagellar motors'. 

Key findings

Dr Beeby and his team were particularly interested in understanding how a class of flagellar motors have evolved to spin with three times higher force than the motors from common bacteria such as E.coli and Salmonella. Motors from the bacterium Campylobacter jejuni and relatives have evolved new motor components that “power-up” their motors. The resulting stronger motors allow swimming through more viscous environments such as mucous. This enables the bacteria to cause diseases in sites that other bacteria cannot reach.

How did the team visualise these fragile motors? By flash-freezing cells so quickly that water molecules didn’t have time to form ice crystals, the group were able to image them in an electron microscope to visualise the intact Campylobacter jejuni flagellar motor.

A key innovation was their development of ultra-thin mutant Campylobacter cells, making them considerably more translucent in the microscope than normal Campylobacter cells, and resulting in unprecedentedly crystal-clear images.

"Like the contraptions of Heath Robinson or Rube Goldberg, evolution is a tinkerer, using the tools at hand to build ever-more elaborate structures." Dr Morgan Beeby Associate Professor in Structural Biology

All molecular machines - flagellar motors included - are made of protein molecules. The team’s molecular model revealed the proteins that supercharged the Campylobacter motor. Intriguingly, the major new protein component is ancestrally related to a widespread family of enzymes - “protein factories” that interconvert food and other molecules in the cell. This finding offers new insights into how molecular machines evolve, and reinforces and emerging understanding of “evolution as tinkerer”, bolting-together pre-existing parts to make new or improve existing (molecular) gadgets.

Dr Beeby said: 'It’s fascinating to finally be able to see how evolution recruited a pre-existing protein molecule - in this case, an enzyme - as a scaffold structure to supercharge the flagellar motors from family of disease-causing gut bacteria. This scaffold enables the motor to have more ‘piston’-like components, leading to greatly-increased rotational power. Like the contraptions of Heath Robinson or Rube Goldberg, evolution is a tinkerer, using the tools at hand to build ever-more elaborate structures.'

Dr Beeby’s team believe this work provides a solid foundation for more studies to understand where other new parts came from, and that there is considerable scope to produce even higher-resolution images in the future. By refining their techniques in future studies, they hope to uncover more about these tiny yet extraordinarily complex wonders of evolutionary engineering.