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Chemotaxis is a kind of taxis, in which bodily cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food (for example, glucose) by swimming towards the highest concentration of food molecules, or to flee from poisons (for example, phenol). In multicellular organisms, chemotaxis is critical in development as well as normal function. In addition, it has been recognized that mechanisms that allow chemotaxis in animals can be subverted during cancer metastasis.
Chemotaxis is called positive, if the movement is in the direction of the higher concentration of the chemical in question and negative, if the direction is opposite.
Bacterial chemotaxis
As if, such as E. coli, have several flagella (4-10 typically). These can rotate in two ways :
- Counter-clockwise rotation aligns the flagella into a single rotating bundle, causing the bacterium to swim in a straight line.
- Clockwise rotation breaks the flagella bundle apart such that each flagellum points in a different direction, causing the bacterium to tumble in place.
Behavior
The overall movement of a bacterium is the result of alternating tumble and swim phases. If one watches a bacterium swimming in a uniform environment, its movement will look like a random walk with relatively straight swims interrupted by random tumbles that reorient the bacterium. Bacteria such as E. coli are unable to choose the direction in which they swim, and are unable to swim in a straight line for more than a few seconds due to rotational diffusion. In other words, bacteria "forget" the direction in which they are going. Given these limitations, it is remarkable that bacteria can direct their motion to find favorable locations with high concentrations of attractants (usually food) and avoid repellents (usually poisons).In the presence of a chemical gradient bacteria will chemotax, or direct their overall motion based on the gradient. If the bacterium senses that it is moving in the correct direction (toward attractant/away from repellent), it will keep swimming in a straight line for longer before tumbling. If it is moving in the incorrect direction, it will tumble sooner and try a new direction at random. In other words, bacteria like E. coli use temporal sensing to decide whether life is getting better or worse. This way, it finds the location with the highest concentration of attractant (usually the source) quite well. Even under very high concentrations, it can still distinguish very small differences in concentration. Fleeing from a repellent works with the same efficiency.
It remains remarkable that this purposeful random walk is a result of simply choosing between two methods of random movement, namely tumbling and straight swimming. In fact, chemotactic responses such as forgetting direction and choosing movements resemble decision-making abilities of higher lifeforms with brains that process sensory data.
The helical nature of the individual flagellar filament is critical for this movement to occur. As such, the protein that makes up the flagellar filament, flagellin, is quite similar among all flagellated bacteria. Vertebrates seem to have taken advantage of this fact by possessing an immune receptor (TLR5) designed to recognize this conserved protein.
As in many instances in biology, there are bacteria that do not follow this rule. Many bacteria, such as Vibrio, are monoflagellated and have a single flagellum at one pole of the cell. Their method of chemotaxis is different. Others possess a single flagellum that is kept inside the cell wall. These bacteria move by spinning the whole cell, which is shaped like a corkscrew.
Signal transduction
A bacterium has three types of transmembrane receptors, for attractants, repellents and periplasmatic proteins. The signals from these receptors are transmitted across the plasma membrane into the cytosol, where che proteins are activated. The che proteins alter the tumbling frequency, and alter the receptors.Flagellum regulation
The proteins CheW and CheA bind to the receptor. The activation of the receptor by an external stimulus causes autophosphorylation in CheA, which in turn phosphorylates CheB and CheY. CheY induces tumbling by interacting with the flagellum protein FliM.Receptor regulation
CheB, which was activated by CheA, is a methylesterase, removes methyl residues from glutamate residues on the cytosolic side of the receptor. It works against CheR, a methyltransferase, which adds methyl residues to the glutamate residues. The more methyl residues are attached to the receptor, the more sensitive the receptor. As the signal from the receptor induces demethylation of the receptor in a feedback loop, the system is continuously adjusted to environmental chemical levels, remaining sensitive for small changes even under extreme chemical concentrations. This regulation allows the bacterium to 'remember' chemical concentrations from the recent past and compare them to those it is currently experiencing, thus 'know' whether it is traveling up or down a gradient.Eukaryotic chemotaxis
Some eukaryotic cells, such as immune cells also move to where they need to be. The mechanism by which eukaryotic cells chemotax is quite different from that in bacteria.
Receptors
For the most part, eukaryotic cells sense the presence of chemotactic stimuli though the use of 7-transmembrane (or serpentine) heterotrimeric G-protein coupled receptors. This class of receptors is huge, representing a significant portion of the genome. Some members of this gene superfamily are used in eyesight (rhodopsins) as well as in olfaction (smelling).Motility
Unlike motility in bacterial chemotaxis, the mechanism by which eukaryotic cells physically move is unclear. There appears to be mechanisms by which an external chemotactic gradient is sensed and turned into an intracellular PIP2 gradient, which results in a gradient in the activation of signaling pathway culminating in the polymerisation of actin filaments, although the details of the signaling pathway is still not totally clear.References
Howard C. Berg (2003), E. coli in motion, Springer-Verlag, NY. ISBN 0387008888
"Chemotaxis"