Have you ever seen a time-lapse video of people on a city block? New York's Times Square, maybe, or the Embarcadero in San Francisco. It's amazing to watch people weave in and out, following their individual paths. This might surprise you, but the microscopic world often resembles that busy city block! Many bacteria are motile, which means that they can propel themselves in a particular direction and even change directions. Motility is very important to bacteria...and sometimes bacterial motility has serious consequences for us.
In the video below (courtesy of the wonderful MicrobiologyBytes Video Library), you can see an incredible range of motile bacteria.
Notice the little elliptical cells, such as those shown at the beginning of the video. Although you can't see it in the video, these cells have one or more whip-like structures called flagella protruding from their cell surface. Here's an image of a bacterial cell and its flagella.
Flagella are really fascinating. A single flagellum is essentially a long helical filament (such as the pink tendrils in the picture above) attached to a rotary motor built from many different proteins and embedded in the bacterial cell surface. When the motor rotates, it spins the filament, which propels the bacterial cell forward. The motor can rotate clockwise or counterclockwise, allowing the bacterial cell to change directions (if you watch the above video again, you can see some examples of bacteria changing directions). In the video below, the whip-like flagella of one bacterial cell are attached to the microscope slide, so the bacterial cell is basically stuck in one place. The flagellar motor rotates, but because the flagella are stuck, the cell just spins wildly, providing a great demonstration of the rotation of the flagellar motor. (Thanks to "microfetish" for posting this video to YouTube).
These flagellar motors can generate some pretty amazing swimming speeds. For example, Helicobacter pylori, which causes gastric ulcers in humans, swims at speeds ranging from 35-55 micrometers per second. Doesn't sound too impressive? Well, a single cell of Helicobacter is about 3 micrometers long. So, this little speedster can cover about 15 body-lengths in one second. By comparison, the current men's world record for the 50 m freestyle is 20.91 seconds, held by a Brazilian named Cesar Cielo (or so Wikipedia tells me). Cesar is 1.96 m tall. So, he swims at about 1.2 body-lengths per second. Maybe our little Helicobacter will wave its flagella at him as it blazes past.
Okay, they're fast, but where are all these bacteria rushing to? Bacterial motility is very sensitive to different kinds of stimulation. For example, bacteria have ways of sensing different chemical compounds, which influence the direction of motility. Some bacteria will move towards higher levels of oxygen, or they will move away from waste compounds. Some bacteria even secrete a compound that attracts members of the same species...kind of a "Hey everybody! Let's get together" bacterial text message.
Motility can also be very important for something called bacterial pathogenicity. And this is where bacterial motility may have serious consequences for us. Remember our speedy friend Helicobacter? I mentioned that this bacterium causes gastric ulcers. Well, flagella and motility are important parts of how this occurs. Helicobacter needs to be motile in order to colonize the gastric mucosa, which is the external layer of your gastrointestinal tract. Using its flagella to propel itself, Helicobacter burrows into the gastric mucosa, where it can cause ulcers. This is just one example of how motility and flagella are important in bacterial infections that afflict humans. Many other pathogenic bacteria use similar strategies. As a result, a number of researchers are studying how to target bacterial flagella with antibiotics or interfere with bacterial motility as a way of treating or preventing these diseases.
Important legal stuff to keep me from getting in trouble with licensing people:
The MicrobiologyBytes video is used here in adherence to the terms of the Creative Commons Attribution-ShareAlike 3.0 Unported License.
MicrobiologyBytes can be found at: http://www.microbiologybytes.com/blog/.
The image of E. coli showing flagella is from the Centers for Disease Control and Prevention's Public Health Image Library.