Dramatic 3D images reveal super-small motors that drive bacteria
When you want to get together with friends or family, chances are you employ a motor. That is to say, you likely get into a car or on some form of public transport to arrive at a meeting point. Bacteria really aren't very different. They have various means of getting around, but they all involve some kind of biological motor — and those motors have just been imaged in dramatic and colorful 3D by researchers at the California Institute of Technology (Caltech).
To image the micromotors, the team employed a technique known as electron cyrotomography. This involves freezing bacterial cells so quickly that the water molecules they contain don't have the time to arrange themselves into ice crystals. Once the cells are locked in their original structure this way, an electron microscope was used to take a bunch of 2D images that were then assembled in such a way that digital 3D images of the motors emerged. The technique was groundbreaking, with Caltech reporting that it was the first time bacteria's biological locomotion machinery has every been imaged in 3D.
"Bacteria are widely considered to be 'simple' cells; however, this assumption is a reflection of our limitations, not theirs," says Grant Jensen, a Caltech professor of biophysics and biology. "In the past, we simply didn't have technology that could reveal the full glory of the nanomachines – huge complexes comprising many copies of a dozen or more unique proteins – that carry out sophisticated functions."
Working with colleagues in the US, UK and Germany, Jensen and his team imaged two different kinds of bacterial motors.
The first, reported in the March 11 issue of Science, was from a soil bacteria known as Myxococcus xanthus and is called the type IVa pilus machine (T4PM). This mechanism lets bacteria move by sending out a long fiber called the pilus. This fiber attaches to a surface and then the bacterial reels itself forward along the tether.
To unravel the fine details of this mechanism, the researchers created a series of mutant cells, each lacking a different component of the T4PM, which they then compared to the intact bacteria so they could map the mechanism. In their observations, they found that the T4PM consists of four interconnected rings. They also found that it's quite powerful.
"In this study, we revealed the beautiful complexity of this machine that may be the strongest motor known in nature. The machine lets M. xanthus, a predatory bacterium, move across a field to form a 'wolf pack' with other M. xanthus cells, and hunt together for other bacteria on which to prey," Jensen says.
The second biological motor that was imaged by the Caltech team involved one that drives the flagellum — a tiny whip-like propellor — which they observed in several different bacteria.
They discovered that there are motors inside the bacteria made from proteins that turn the flagellum. What's more, these protein structures were often found quite far from the flagellum, which means they could generate significant torque. It's kind of like a small rotor on a fishing boat, versus a large one on a yacht. Their work with the flagellum motors was published in the March 29 issue of the journal PNAS.
"These two studies establish a technique for solving the complete structures of large macromolecular complexes in situ, or inside intact cells," Jensen says. "Our electron cryotomography technique is a good solution because it can be used to look at the whole cell, providing a complete picture of the architecture and location of these structures."
To see exactly how pili help move bacteria along with their motor-powered grappling-hook method, check out this video.