Motility is, simply put, the ability of an organism to move by itself. In this article, we’ll be discussing the motility of bacteria. Not all bacteria are capable of motion. The ones that are motile use whip-like structures called flagella to propel themselves through an environment, or small, hairlike structures called pili to move along a surface. They typically move in response to chemical signals (closer to nutrients, farther from dangerous substances) or toward each other. Bacteria follow different patterns of motion, such as tumbling, gliding, or darting.

Flagella can be arranged on a cell in different ways. Monotrichous means that there is a singular flagellum, placed only on one end. Lophotrichous flagella are still situated on one end, but multiple flagella instead of only one. Amphitrichous involves multiple flagella on both ends. Peritrichous flagella are placed pretty much all over the cell.

There are various types of pili; some are used for conjugation (genetic transfer between two cells) or attachment to surfaces. Type IV pili are used for motility. As this type of motility is less steady, it is called twitching motility.

If we wanted to determine whether a species of bacteria is motile, we could conduct a (very aptly named) motility test! We would take a test tube containing solid motility medium (gel) and use an inoculating needle to transfer bacteria two-thirds of the way down into the gel. After one or two days, we would come back to check the results. The path where we inoculated the medium (and where the microbes grew) should look red. The color change is caused by a reaction between a chemical compound in the medium and the metabolic process of the bacteria. If the redness is restricted to the path, it is negative for motility; our microbe cannot move past the path where it was placed. If the redness has spread throughout the tube, this indicates a positive motility result; the microbe can move throughout the media!