Bacterial motility

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Most Bacteria have the ability to move from one place to another in at least some part of their life cycle. The most common and best-understood form of motility is that driven by flagella, but many mechanisms of motility are used by Bacteria, and in some cases a specie may be capable of more than one type of motility.

Although only well-studied in a few flagellated organisms, the che gene signaling pathway for chemotaxis are widespread in Bacteria, and the same process probably regulates chemotaxis in other organisms regardless of their method of motility.

Flagella

flagella

A wide range of Bacteria are motile via flagella. Flagella are proton-gradient-driven helical propellers, allowing the organism to “swim” through an aqueous environment. Cells can have one to many flagella, located at one or both ends or distributed all over. A rare arrangement is to have flagella only on one side of a rod-shaped cell - these cells swim sideways, counter-rotating like a propeller.

Some flagellated organisms are curved (vibrios) or spiral-shaped (spirilla). This increases the efficiency of flagellated motility, despite the increase in surface area. Viscous resistance on the rotating flagella cause a counter-rotation of the cell body; if the cell is properly curved, this lost energy is recaptured by turning the cell into a screw.

spirilla

Cells 'run' (move continuously in a more-or-less straight line) when the flagella turn one direction, and 'twiddle' (tumble) when they turn the other direction. The length of a run is dependent of whether the cell is moving in the desired direction - long runs if so, short runs if not. Twiddling reorients the cell randomly between runs. The result in a directed random walk - a fairly efficient way to get where you want to go.

Spirilla are little different - they typically have polar flagella, and "twiddle and run" like other flagellated organisms, but usually switching from running the flagella and one end of the cell to the other end of the cell after each twiddle. They can also switch back and forth between running the flagella at either end of the cell without twiddling, resulting in the cell running directly back and forth. They also use this ability to quickly reverse directions when they run into something, or come into abrupt contact with a repellant.

Gliding

Gliding motility is accomplished by at least two fundamentally different mechanisms.

Cyanobacteria, Chloroflexi, Thiotrichs and probably Myxobacteria, and many eukaryotic algae, glide using a mechanism that involves the secretion of polysaccharides from pores on cell surface. Hydration of the polysaccharide as it emerges into the aqueous environment causes it to expand dramatically and provides a reactive force much like a rocket engine. Unicellular gliders usually have the pores at each end of the cells (which are typically rods). Filamentous gliders have these pores along the leading and trailing edges of individual cells, oriented fore and aft.

gliding1

How cells control which pores to activate, so that the cells move in one direction or the other, is not known. Nor is the coordination of motility between cells of a filament understood. Gliding leaves a polysaccharide 'slime' trail stuck to a substrate or hanging in solution, and is an efficient a way to move over the surface of a solid material as well as through liquid media.

The Bacteroids glide using a very different and poorly understood mechanism. Adhesins on the surface of the cells seem to move uniformly from one end of the cell to the other. Presumably the adhesins are internalized upon reaching the trailing end of the cell, and reemerge at the leading edge. Think of the tracks on a bulldozer, a conveyor belt, or an escalator. These organisms can only glide if in contact with a surface. It has been proposed that gliding in Myxobacteria may be similar, except that the surface adhesins follow a helical path along the surface of the cell, guided by the cytoskeleton.

gliding2

Gliding in Mycoplasma is poorly understood, and may be based on two different unique mechanisms. Rapid gliding in some Mycoplasma species may be the result of protein “legs” on the leading appendage of these organisms. Slow gliding in other Mycoplasma species may involve surface adhesins and “inchworm” extension and contraction of the leading appendage.

Twitching

twitching

A few organisms, such as Myxobacteria (which also glide) and some species of Pseudomonas (which can also produce flagella), Neisseria, Nostoc and Clostridium can move across surfaces using retractable pili. Think of this as grappling-hook motility; the cell extends a pilus (type IV, where it has been determined) in the direction it wants to move, the end of the pilus attaches to the substrate, then retraction of the pilus pulls the cell forward, generally a few cells lengths at a time. Each pull looks like a 'twitch', thus the name. Some types of cells can produce many pili simultaneously, so that the cell can more usually move more-or-less smoothly forward, looking a lot like gliding, for which this is often mistaken. In order to change direction, the cell has to disassemble the pilus apparatus from one end and reassemble it at the other - and, again, the mechanism for this is not at all understood.

Gas vacuoles

gas vacuoles
The hexagonal forms inside the cytoplasm of this cyanobacterium are the gas vesicles. These actively dividing cells are Microscystis sp.(Source: A. E. Walsby, 1994. Microbiol. Rev. 58:94-144)

Many aquatic phototrophs move themselves up and down in the water column by fine-tuning their buoyancy using gas vacuoles. These can be bound by a lipid membrane similar to the cytoplasmic membrane, or by a protein layer. Gas vacuoles usually contain CO2 generated by metabolism. Gas vacuoles are very rigid structures, and do not compress or expand significantly over a wide range of pressures; this helps simplify maintaining constant buoyancy. Otherwise, the gas expansion during ascent or compression during descent would require the organisms to constantly adjust their buoyancy, a problem well known to SCUBA divers.

Spirochaete motility

Spirochaetes move by rotation of periplasmic flagella. This method of motility is therefore structurally related to flagellar motility, but is mechanistically very different. Rotation of the periplasmic flagella cause the rigid helical cell body to rotate within the outer membrane. Viscous resistance with the surrounding media prevents the outer membrane from spinning freely. As a result, the shape of the cell relative to the surrounding media forms a rotation corkscrew, as so drives the cell forward.

spirochaetes

Not all spirochaetes are helical; many are flattened waves. The same mechanism caused the shape of the cell to wave and progress forward much in the same way that a snake moves forward on flat ground. In some cases, spirochaetes are bent or curved at the ends (see Leptospira, above) to improve motility through semisolid environments, such as the interstitial spaces between animal host cells.

Spirochaetes run when the two terminal flagella rotate in opposite directions (i.e. together, since they are from opposite ends of the cell). The cell can switch the direction of motion by switching the rotation of both flagella simultaneously. If the switch is not simultaneous, the cell flexes while the flagella rotate out-of-sync, analogous to the twiddles of other bacteria.

Spiroplasma motility

Spiroplasma

Propagation of a kink along a cell of Spiroplasma. : Schlomo Trachtenberg, chapter 6: Mycoplasmas: Molecular Biology, Pathogenicity and Strategies for Control, By Alain Blanchard, Glenn Browning (Contributor Alain Blanchard)

Spiroplasma is a relative of Mycoplasma. Spiroplasma cells are helical, and move by a novel mechanism based on changes in shape of its internal cytoskeleton. This cytoskeleton is a flat ribbon composed of 14 fibrils (7 pairs) that is fixed to the inside surface of the cells along the midline of the cell spiral. Independent contraction of the fibrils in this cytoskeletal ribbon can be used to contract or expand the helical shape of the cell, or even reverse the handedness of the helix, at any point along the length of the cell. Motility is driven by moving a stretched or contracted region of the cell, or a kink produced by a short region of reversed handedness, in a wave from one end of the cell to the other; viscous drag on this irregularity in the helix results in rotation of the cell, which drives it forward.