Imagine that you're mostly made up of water. Well, you are, so you don't have to try very hard. But now imagine that, instead of keratin-rich skin, the only thing that holds you together is a thin bilayer of lipids--molecules which are most familiar to most people as components of fat. Finally, imagine that you're only 10 microns (.000010 meters) wide.
Now you have a problem. You're much, much smaller than most water droplets. Water droplets form because of surface tension, which you're familiar with if you've ever watched rain on the window. The water molecules organize themselves in such a way that they can rise above the glass. These forces are so strong that if you fill a straw with water and put your finger over the top, the water won't run out the bottom, even though the water column is exerting a lot of pressure on the tiny hemisphere of surface-tension-secured water at the bottom. The narrower the straw, the higher the water column it can hold.
So if you're only 10 microns wide, mostly water, and held together by a flexible lipid-lined bag, you have a serious problem which cells have confronted since their appearance in the primordial soup. Surface tension will easily force you to collapse into a sphere. But cells are rarely spherical: they can be cylindrical (intestine lining), amoeboid (immune cells), long and slender (muscle cells), covered with protrusions (neurons) or more. Moreover, cells can crawl, drastically changing their shape in order to pull themselves along. Our job is to ask them how they do it.
The answer starts with the cell's skeleton, a fibrous network made up of filaments of varying thickness. Motility is mostly the jurisdiction of actin filaments, whose remarkable strength helps resist the forces of surface tension.
Actin filaments are made up of protein monomers--individual building blocks that can be linked together like Legos to form long or short chains. Other proteins in the cell bind actin filaments to link them together, stop their growth, accelerate their growth, or chop them up. Yet other proteins bind to actin monomers to regulate the biochemical equilibrium between monomers and filaments. In other words, a cell's skeleton is nothing like our skeleton in that it changes structure very quickly.
Our two favorite pathogenic bacteria--Listeria monocytogenes and Shigella flexneri are excellent cell biologists. They understand actin-based motility so well that when they invade into the interior of cells, they hijack the cell's actin-based motility system to facilitate their own spread into adjacent cells. Since their motility is easier to measure than that of a cell, we spy on them as they've spied on cells, trying to learn what they know.
Here's what we and other labs have figured out. Each bacterium produces a single protein (ActA for Listeria or IcsA for Shigella) which encourages the growth of new actin filaments at the bacterial surface. As these filaments grow (polymerize), they exert force on the bacterium, pushing it forward from the outside, much as actin filaments push their cells forward from the inside. The bacterium rockets forward, leaving behind a comet tail of short, highly networked actin-filaments which shrink (depolymerize) over time. You can see one of these tails in the movie at the top of the page.
Think of it like a jet contrail (800k file). Addition of new material only occurs at the back of the 'organism,' and the organism causes that addition. The tail is stationary with respect to its environment (the cytoplasm or the atmosphere). And finally, that environment governs how fast the tail falls apart (host proteins or wind).
In an incredible example of pathogenic evolution, these bacteria become the terrorists of the cell, hijacking it's transportation mode to ram them into the cell's membrane. The bacterium then protrudes into an adjacent cell, and, upon escaping into the cell's neighbor, continues dividing and spreading to other cells. By never exiting cytoplasm, the bacterium avoids the commandos of the immune system: large, crawling cells called macrophages which are constantly policing the body for harmful intruders to eat.
By studying how these pathogens move, we deepen our understanding of many biological problems. It's a two-for-one deal. On the bacterial side, we can learn about the relationship between host and pathogen, helping modern medicine combat the diseases caused by these organisms. Effects of listeriosis and shigellosis include diarrhea, meningitis, spontaneous abortions, and even death for immunocompromised individuals (children, the elderly, AIDS patients). On the host cellular side, we can learn how cells use their skeletons to change shape in developing tissues, how they move in the development of a new organism, how wounds are healed by migrating cells, how metastatic tumor cells crawl into the blood, and many other physiological processes.
How cells hold or change their shape may seem at first like an esoteric question. Yet to the cell, it's anything but. And since cells make up every living thing on this planet, we think listening to them is worth the effort.