Location, location, location. So far, we have focused on how your cell can take the best advantage of its spot in the world. But why not find a better spot? Some cells live stationary lives, attached to a surface or embedded in a biofilm. Many, though, are explorers, using an impressive variety of techniques to move through their environments, finding advantages in places with more food or fewer competitors. In this chapter, we explore how your cell might make a mobile home.
Most bacteria and archaea live in liquid, so motility means swimming. When you are the size of a cell, though, the backstroke does not get you very far. A rough measure called the Reynolds number describes the relative influence of inertia and viscosity on liquid flow, and this parameter scales with an organism’s size. In the low-Reynolds-number world of microbes, inertia is virtually nonexistent. When a rod-shaped bacterium stops swimming, it gets to coast only about the diameter of a hydrogen atom (~1 Å) before stopping. In this molasses-like environment, rotary propellers work much better than paddles.
Nearly all bacteria that swim use the same propeller: a rotary motor embedded in their envelope that spins a long helical fiber called a flagellum (⇩). A universal joint called the hook connects the filament to the motor, translating the rotation. Flagella, like the one on this Campylobacter jejuni, are typically many times longer than the cell and take the form of a three-dimensional wave. The filament is highly dynamic, as you would expect, and throughout this book, you will see examples caught in various conformations: straight, curved, or in a typical loose helical waveform like this. (You can watch videos of Escherichia coli swimming with flagella on Howard Berg’s website.)