Many bacteria are motile and they can migrate by utilizing mechanisms based on different principles. Motility mechanisms have been developed in bacteria in order for them to be to be attracted or repelled by certain stimuli. Motility is a feature, which is used for characterization and identification of bacteria and methods have, therefore, been developed to detect motility. Bacterial locomotion must be distinguished from so-called Brownian molecular motion, which is random and depends on the thermal motion of water molecules, which can push the bacteria so that it is perceived as if the bacteria move under the microscope.
Swimming movement by means of flagella
Bacteria can have flagella, which may be one or several in number, and which act as propellers and allow the bacteria to move in a predetermined direction. Very simplified, you can say that the flagellum consists of a moving filament, which is linked to a molecular engine (basal body) in the bacterial envelope via a hook. The flagellum consists of many subunits of the protein flagellin, which form a hollow and flexible cylinder. The molecular engine, which is built up by protein subunits, is driven by the proton gradient (or a Na+ ion gradient) over the cell membrane and can get the filaments to rotate clockwise or counterclockwise. This rotation causes the bacteria to swim in a certain direction and to tumble, respectively. The direction is dependent on outer stimuli and during tumbling, the bacteria can change direction of the movement.
Bacteria within the phylum Spirochaetes have so-called periplasmic flagella (endoflagella or axial filaments), which are localized in the periplasmic space between the cell membrane and the outer membrane. This arrangement result in a screw like motility (or flat-wave motility), which makes it possible for these bacteria to move in highly viscous material (like mucus).
Twitching motility by means of type IV pili (fimbria)
Bacteria which have so-called type IV pili can move by using the external ends of the pili, which have hooks, and can adhere to a solid substrates like the surface which the bacteria colonize or to other bacteria. When the pilus contracts, the bacteria are pulled forward. Movement produced by type IV pili is typically jerky, and thus it is simply called twitching motility. Pseudomonas aeruginosa has this twitching motility.
Gliding motility on surfaces
Some bacteria can glide on wet surfaces, but the molecular mechanisms are incompletely understood. Examples of bacteria, which have this ability are: members of the phylum Cyanobacteria and the genera Flavobacteria and Mycoplasma.
Some pathogenic bacteria can move inside the host cell by using its cytoskeleton. The cytoskeleton is normally used to move organelles inside the eukaryotic cell. By stimulating actin polymerization at one of its poles, these bacteria can form a kind of tail, which pushes them through the cytoplasm of the host cell. Examples of bacteria that can use this mechanism are species within the genera Burkholderia, Listeria, Rickettsia and Shigella.
Motility tests can be performed in test tubes containing a semi-solid medium, i.e a medium with a low concentration of agar. The medium is inoculated with a plastic loop, which is inserted into the agar and pulled up again. If the bacterium is motile, the medium in the whole tube will turn turbid, but if it is non-motile turbidity will only be obtained where the plastic loop was inserted. The picture shows a negative control (A), which consists of Staphylococcus aureus subsp. aureus (non-motile) and Escherichia coli (B), which is motile.