Tuesday, December 14, 2010

Twitchy

The most common image that people have of how bacteria move is of a little bug swimming along using its flagella. So busy!

video

But the bacteria have other means of getting around—one of the more amusing ways of moving is called “twitching motility.” Cells that move by twitching motility don’t have a flagellum. Instead, they produce a thing called a "pilus." This is kind of like the telescoping radio antenna you see on a car: it’s a thin shaft of protein that can extend from the cell, then retract back--here's Pseudomonas pulling in some pili:

video

The end of the pilus has a sticky protein on it, so to continue our metaphor, the pilus is like a telescoping antenna with a blob of crazy glue on the end. A twitching cell extends its pilus, sticks it to something, then draws it back in. Here's Pseudomonas grabbing onto a hair, then pulling itself along:

video

Rather than smoothly swimming, the motion is herky-jerky. Or, twitching. So, it's called twitching motility!


This is how I’ve always pictured twitching motility: a cell lying down, and moving in a linear motion. But, what if the cell is standing on end? If it just made one pilus, it would pogo up and down. But if it made a few pili, it would sort of stagger—go up, then go down a little off to one side; go up, then go down a little off in another direction, depending on how poorly synchronized the pili were. A twitching cell standing on end would “walk,” but it would be a drunkard’s walk.


It turns out that there are situations where a drunkard’s walk is actually very useful. An army of drunkards, all starting at the same place, will effectively “explore” every nook and cranny of an area. If the “drunkards” are twitching bacteria, standing on end, then a population of bacteria can effectively, if blindly, explore every bit of an area, and if there’s a particularly good place to set up shop, at least one individual bacterium is bound to find it (panel A).

In contrast, the bacteria who are “lying down” and moving by twitching motility will travel farther, but won’t cover an area as thoroughly (panel B).

This isn’t just an amusing vision of staggering bacteria—a set of experiments led by Gerard C. L. Wong showed that certain types of bacteria actually use this method to effectively explore an area. Using microscopes rigged with cameras and computers, he was able to track individual bacteria; he found that the “staggerers” were much better at covering an area than the twitchers who were lying down, or those lacking pili altogether.


The bacteria that lacked the ability to make pili never really explored their environment, so they ended up crowded—a situation that makes it hard for them to get the food they need.

Bacteria that could make pili and walk around after dividing were nicely spaced out, with plenty of elbow room—and access to food.

The need to get up and explore seems to be built in to the bacterial program: Wong’s group watched hundreds of individual bacteria divide, and most often division resulted in one cell that was lying down and another cell that was “standing,” ready to get up on its pili and stagger around. If things were just due to chance, you'd expect far more freshly divided bacteria to be lying down--not the most efficient way to disperse. It seems that roving is in the genes.


This is not an earth-shattering discovery. However it does provide some insight into how bacteria explore and exploit their environment, and how wanderlust seems to be genetically programmed. It also provides the amusing mental image of a cell, twitching and staggering around like a Dickensian drunkard looking for a place to sleep it off.


Maxsim L. Gibiansky, Jacinta C. Conrad, Fan Jin, Vernita D. Gordon, Dominick A. Motto, Margie A. Mathewson, Wiktor G. Stopka, Daria C. Zelasko, Joshua D. Shrout, Gerard C. L. Wong (2010) Bacteria Use Type IV Pili to Walk Upright and Detach from Surfaces
. Science 330:197.

The movies are from Howard Berg's lab's excellent website.

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