Screwy Locomotion: the Spirochete

How do spirochete bacteria move?

Getting around isn't easy. Some of our greatest technological advancements have been in locomotion. Taming, then riding, horses; railroads, automobiles, airplanes. Microorganisms have been around for a long time, and while flying may be easy for them, getting through thicker media is not, nor is steering. The classic form of bacterial motion is with an outboard motor- the flagellum. The prototypical bacterium E. coli has several flagella sprinkled around its surface. Each flagellum is slightly helical, thus forming a languid sort of propeller, which if turned along its helical axis, (at roughly 6,000 rpm), can propel the bacterium through watery media. Turning multiple flagella in this same direction (counter-clockwise) encourages them all to entangle coherently and unite into a bundle. It turns out, however, that bacteria can easily switch their motors to the opposite direction, which causes the flagella to separate, and also to flail about, (since for a left-handed helix, this is the "wrong" direction), sending the cell in random directions.

A typical bacterium with multiple flagella, which will cooperate in forming a bundle when all turned in the same direction, consonant with their helicity (i.e. counter-clockwise).

These are the two steering options for most bacteria- forward or flop about. And this choice is made all the time by typical bacteria, which can sense good things in front (keep swimming forward), or sense bad things in front / good things elsewhere (flail about for a second, before resuming swimming). The flagellar base, where the motor resides, uses both ATP and the proton motive force (i.e. protons that were pumped out by cellular respiration, or the breakdown of food). The protons drive the motor, and ATP drives the construction of the flagellum, which is itself a very complicated dance of self-organization, built on the foundation of an extrusion/injection system also used by pathogenic bacteria to inject things into their targets.

Animated video describing how the flagellum and its base are constructed.

But sometimes a bacterium really needs to get somewhere badly, and is faced with viscous fluids, perhaps inside other organisms, or put out by them to defend themselves. One human defense mechanism is a DNA net thrown out by neutrophils, a type of white blood cell. Spirochetes have come up with an ingenious (by evolution, anyway!) solution- the inboard motor. This is not a motor sticking out of the bottom, but a motor fully enclosed within the cell wall of the bacterium.

Choice of directions (small forward or back arrows) that are dictated by the rotation of the flagella (blue). One set of flagella originate at the rear, and a second set originates at the front. Only if they turn in opposite directions (top two panels) does the spirochete swim coherently, either forward or back. 

How can that work? It is an interesting story. Spirochetes, as their name implies, are corkscrews in shape. In mutants lacking flagella, they instead relax to a normal bacterial rod shape. So they have flagella, but these are positioned inside the cell wall, in the periplasmic space. Indeed they form the central axis around which the corkscrew rotates, with one set of (approximately ten) flagella coming from the rear and another set from the front, each ending up around the middle. If each set rotates as hard as it can, they drive their respective ends to counter-rotate, in reaction. If the front motors (of which there are several) turn their flagella counterclockwise, as viewed from the back, they will, in reaction, drive (and bend) the nose into a clockwise orientation. If the back set of motors run clockwise, driving their flagella counterclockwise (also as seen from the back), then the rear part of the bacterium counter-rotates in clockwise fashion, and the coordinate action drives spiral bending and an overall drilling motion forward.

Video of a non-spirochete bacterium with its flagellum stick to the slide, causing the tail to wag the dog.

Video of spirochete bacteria in motion.

On the other hand, if the motors on the opposite ends of the bacterium go in the same direction, then the flagella induce opposite, instead of coordinate, counter-rotations, and the bacterium doesn't tumble randomly, as normal bacteria do, but contorts and flexes in the middle, with a similar re-orienting effect. This ability incidentally shows the remarkable toughness of these bacteria, considering the lipid bilayer nature of their key protective membranes. These bacteria can also easily reverse direction, by sending both sets of motors in reverse, operating very much like little drills. How this exquisite coordination works has not yet been worked out, however.

Reconstruction, drawn from electron microscopy, of one end of a spirochete, showing the motor orientations, the sharp hook/base of the flagellum, the membrane and cell wall structure, and one of the signaling proteins (MCP), which transmits  a sensory signal to dictate the direction of motor rotation.

One thing that is known, however, is that spirochete motors are massive- almost twice the size of E. coli motors, with special outside hooks to propagate power through the tight turn inside the periplasmic space. It is interesting that these motors can be scaled up in size, with more subunits, and more proton ports for power, as if they were just getting more cylinders in a (fossil fuel-burning) car engine.

Structure of the Borrelia flagellar motor, showing the stator (blue), which is attached to the membrane and stabilized against rotation; the rotor (yellow spokes and teal C-ring), and the gateway ATPase complex which unfolds and transmits the structural components (proteins) into the central channel from which they build the machine.

All this is in service of getting through messy, gelatinous material. The model for most of this work is the spirochete responsible for Lyme disease. The characteristic red ring seen in that infection is thought to track the progress of the spirochete outward and away from the original tick bite site, in relation to the immune system catching up via inflammation. But such viscous environments are quite common in the organic muck of the biosphere, including biofilms established by other bacteria. So the evolutionary rationale for the superpowers of spirochetes is probably quite ancient.

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