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The term chemotaxis is defined as the process in which organisms, both unicellular and multicellular, move toward or away from a chemical source that is present in the environment. Depending on their behavior to the chemical, chemotaxis can be positive, if cellular movements go toward higher concentration of the chemical, or negative, if they attempt to move away. In the first case, the chemical is called chemoattractant, in the second case, chemorepellent. Therefore, whereas the outcome of positive chemotaxis used to be the accumulation of organisms or cells in regions with higher concentration of chemoattractant, the outcome of negative chemotaxis would usually be separation and dispersal of them from the chemorepellent. Generally, this behavior is a consequence of using the chemoattractant as a nutrient, and because the chemorepellent are excretion product or toxic. However, chemicals are not the only stimuli that cells and organisms can sense. They also are able to detect others stimuli like light, temperature, touch, electric fields, etc.

Bacteria have the capacity to respond to a great diversity of chemical stimuli. Usually, there are families of substances that only act as chemoattractants and others that only do it as chemorepelents, but this is not a fixed rule. For example, E. coli has positive chemotaxis to sugars, oxygen, weak bases, dipeptids, whereas it present negative chemotaxis to alcohols, weak organic acids, inorganic ions and extreme extracellular pH values. As for amino acids, exist a division in their action and they exhibit a gradation. E. coli is strongly attracted to L-alanine, L-asparagine, L-aspatate, L-cystein, L-glutamate, glycine and L-serine, but is repelled from hydrophobic amino acids like L-isoleucine, L-leucine, L-phenylalanine, L-tryptophan and L-valine. Chemotactic response of E. coli to amino acids involves two MCPs (methyl-accepting chemotaxis proteins), which are the products of tar and tsr genes. In this case, E. coli and Salmonella are very similar, whereas B. subtillis and Pseudomonas aeruginosa are positive chemotatic to the common twenty L-amino acids. Taxis towards L-amino acids depends on the nitrogen availability.

Chemotaxis response is characterized by great sensitivity, huge dynamism and precise adaptation, that allow E. coli to amplify the sensed stimuli up to 50-fold. In bacteria like E. coli and Salmonella, some chemotactic stimuli bind directly to chemotaxis-specific receptors, whereas others bind first to a primary receptor, which then interact with the respective receptor. There are different genes involved in taxis: those that encodes chemotatic receptors for specific components (receptors, transducers and Che genes). E. coli tactic receptors are the product of five partially redundant genes: tsr, tar, trg, tap and aer., that sense a wide variety of chemicals. Vibrio cholerae has 43, H. salinarum has 3 subfamilies of MCPs, M. xanthus only one, and B. subtilis has ten.

The receptor complex is composed by MCP-CheW-CheA. Usually, the MCPs are clustered at the bacterial poles (one or two). At least, MCPs have three main functions related to bacterial chemotaxis: they bind the ligands, they transduce the chemotaxos signal across the cytoplasmic membrane and they undergo methylation or demethylation (adaptation process). Received information is transduced by a group of proteins that are coordinated by phosphorilation-dephosphorilation reactions (CheW, CheA and CheY), and act on the flagellar motor changing its direction of rotation (clockwise or counterclockwise). The methylation-demethylation reactions (focusing in Tar MCP) take place in a carboxilic group of glutamate and glutamine residues as part of a chain reaction, which is carry out by the enzyme methyltransferase CheR, and the opposite reaction is carried out by the enzyme methylesterase CheB. As for chemorepellent, in most cases, the ligands bind to specific receptor, although sometimes this chemicals cause cellular perturbations that directly act on MCPs.

In bacteria, cell motility are a consequence of flagella rotation (in flagellated bacteria), which let them move in liquid mediums, and sometimes in solid mediums (agar plates). The name of these processes are swimming and swarming. However, there are others types of movements like: gliding movements (when cells have no cilium or flagella), twitching movements (intermittent, directionless and uncoordinated movement). Pseudomonas motility is due to the rotation of an only flagellum, H. salinarum motility is caused by a bunch of polarized flagella, but Escherichia coli, Salmonella y Bacillus subtillis have lots of flagella that are randomly spread out all over the membrane. Most of the time all flagella rotate in the same direction, provoking straight movements called run, but when the direction changes the bacteria began to go round (tumbles).

Chemotaxis is a widespread phenomenon in organisms, because their natural tendency is approach to beneficial environments, according to their requirements, and avoid from damaging ones. Although it seems a very basic and easy process that only involves changes in speed and direction of rotation, actually it is a complex behavior that implicates the continuous integration of extracellular stimuli and their intracellular coordination to respond fairly accurately.

Many bacteria use the chemotaxis to interact with both animal and plant hosts and chemotaxis plays an important role in the fitness and virulence of bacteria. Most plant-associated bacteria have swimming motility, which let them reaches plant tissues and causes invasion and colonization. Pathogenic bacterium is specifically attracted to diverse amino acids, sugars, aromatics, secondary metabolites and organic acids, natural component of plant exudates, that may act like chemoattractants. For example, sugars attract Rizhobium leguminosarum, Azospirullum brasilense and Agrobacterium tumefaciens, but not Erwinia amylovora or Pseudomonas fluorescens. Thus, chemotactic responses may be differentially selected traits that confer adaptation to various host and ecological conditions.





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