Studying bacterial locomotion, one wouldn’t think that such erratic, but purposeful, movement would cause controversy outside the normal confines of academic meetings. Yet, the bacterial flagellum has been at the center of the thinly veiled creationism movement called intelligent design. Subscribers to this belief system have erroneously postulated that the flagellar motor system is “irreducibly complex” and could not have come about through Darwinian evolutionary mechanisms.
However, now researchers at Imperial College London have utilized cutting-edge imaging techniques that have produced incredible images of bacterial motor parts in extremely high resolution. A comparison of microbial motors from different species has led the investigators to speculate an even older origin than previously thought. It is doubtful these findings will sway the opinion of its detractors, yet they do make it extremely more difficult for them to make their case.
Interestingly, despite motors in diverse bacteria having the same core structure, different bacteria vary widely in their swimming power. For instance, Campylobacter jejuni, which causes food poisoning, can swim powerfully enough to bore through the mucus that lines the gut, an environment too thick and sticky for other bacteria to push through. Still, before the release of the current study data, the reason for these differences in swimming ability had eluded scientists for quite some time.
Bacterial flagellar motors work on a rotational mechanism, spinning their long flagellar tail to produce a helical propeller-like motion. The researchers found that stronger swimmers evolved by adding extra parts to their motors, making more powerful motors that increased the turning force, or torque.
To acquire the stunning structural images of the flagellar motors, the Imperial College team utilized a method called electron cryotomography to freeze the bacteria rapidly to –180°C, thus preventing ice crystals from forming that would break the structure apart. This allowed the researchers to capture the flash-frozen motor from all angles and build a high-resolution, three-dimensional model.
The findings from this study were published recently in the Proceedings of the National Academy of Science in an article entitled “Diverse High-Torque Bacterial Flagellar Motors Assemble Wider Stator Rings Using a Conserved Protein Scaffold.”
“For the first time, we have been able to see and explain how these nanoscale molecular machines have evolved in bacteria to colonize new environments,” explained lead study author Morgan Beeby, Ph.D., lecturer in structural biology at Imperial College London. “It's a fascinating insight into the awe-inspiring diversity of life that has evolved on Earth, and also presents possible drug targets. We may be able to design drugs that specifically sabotage the flagella only in targeted bacterial species.”
Surprisingly, the investigators found that not all bacteria need to be so powerful and swim through such viscous environments as stomach mucus. Instead, the team looked at another bacterial species from the genus Vibrio—members of which cause human disease such as cholera—and found that this bacteria has evolved a motor with only intermediate power.
“Entire branches of the bacterial family tree have evolved motors with different torques, leading to a diversity of species each geared to their own environment,” Dr. Beeby noted.
While looking at distantly related bacteria from different branches of the evolutionary tree, the researchers speculated that the ability to alter torque in this way may have evolved up to two billion years ago. The team is now investigating how and when the evolutionary steps that altered motor torque could have happened.