The many ways bacteria move

Considering the many diverse environments that bacteria live in, it isn't surprising that they have many different ways of moving. Swimming, swarming, gliding, twitching, floating: these aren't just different ways of describing the same movement, these are specifically different mechanisms that bacteria use to manouvre themselves across surfaces, through liquids and towards their preferred environments and food sources.

Most bacteria use their flagella to swim. These are long proteinous threads which look a little like tentacles. Some bacteria just have on of them, while others have most of their body covered in flagella, in order to provide maximum propulsion:

Image from the National Centre for Science Education

Flagella act like a rotory motor at the back of the bacteria, propelling it forward. Flagella based movement is usually referred to as swimming, and is the best studied of all bacterial movements. When all the flagella are moving in the same direction (usually clockwise) they shoot the bacteria forward. In order to stop, the flagella are sent spinning anticlockwise, breaking up the shape and leading to the bacteria tumbling around directionlessly, before the flagella are activated again.

However many bacteria are capible of movement without using flagella, such as the plant and insect pathogen Spiroplasma which moves due to the action of internal filaments. The contractile cytoskeleton is thought to function as a linear motor, meaning the bacteria moves along like a swimmer doing butterfly stroke, through generating a moving kink through the cell, propelling it forward.

Bacteria can also use smaller protrusions called pili, which stick out from the cell surface. There are several different kinds of pili which have different functions (type three are used for pathogenicity, as covered here) and type four pili are commonly used for movement. This is often referred to as 'twitching' as it results in jerky movement. Pili are also used to glide accross surfaces, or even to stand up and walk across them, on little pili legs!

Gliding motility does not always require pili; the bacteria Flavobacterium uses molecules called adhesins to grip to a surface and slide along it. The movement is powered by motors of Gld protein in the bacterial cell wall, as shown in the diagram below (from the reference):



Myococcus Xanthus (which I covered in further detail here) is more likely to use a motor within the cell to create a rotational movement (although movement via polysacharide secretion has also been suggested). The cytoplasmic AglZ protein is thought to act as the motor, as it remains stationary with respect to the surface as the cells glide accross it. This is shown below (image from the reference):


In addition to these active forms of movement, some bacteria also proceed more passively, without using much of their own energy. Aquatic bacteria can use internal gas vesicles in order to rise to the surface (to be closer to sunlight or nutrients). Other bacteria spread simply by growing, pushing bacteria near the surface of the colony into new terratory by all the bacterial growth around it. One of my favourite is the method used by intracellular parasites such as Listeria monocytogenes which polymerises the actin molecules of it's host behind it to travel through the human cell, leaving little superman trails of actin behind it.

There is a huge diversity in the way bacteria move, and interestingly, many of these strategies are also connected to other intracellular processes, after all the mechanism for moving yourself along a surface need not be different to the mechanism for moving molecules around directly under the cell wall. Studying these different ways helps to give an appreciation of the world bacteria live in, and how they organise themselves to survive it.

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Jarrell KF, & McBride MJ (2008). The surprisingly diverse ways that prokaryotes move. Nature reviews. Microbiology, 6 (6), 466-76 PMID: 18461074

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