Chemotaxis Clause Samples

Chemotaxis. Bacteria can have bi-directional flagellar motors which permit changes in the direction of flagellar rotation. Tumbling results from clockwise (CW) flagellar rotation, and running is caused by counter-clockwise (CCW) rotation (124). Bacteria will alter the direction of locomotion in response to the presence or absence of attractants and repellents in the environment, called chemotaxis. Examples of attractants are primarily nutrients, including amino acids, sugars, and dipeptides, though the interspecies quorum sensing signal AI-2 and pyrimidines also serve as attractants for E. coli (150, 151). Some examples of repellents include fatty acids, aliphatic alcohols, aromatic compounds, inorganic ions, and mercaptans (152). Chemotaxis affects the migration of swimming cells which move forward (run) and change direction (tumble) in response to gradients of attractants or repellents. Chemotactic behavior causes seemingly chaotic swimming patterns that result from rapid succession between running and tumbling. However, motility is rarely random. Bacteria have evolved intricate and incredibly sensitive chemosensory systems which promote their survival in various environments (110, 153, 154). The chemotactic system that affects swimming motility in Gram-negative enteric bacteria is an atypical two component system that affects the phosphorylation state of the response regulator (RR) CheY. This system has been best-studied in E. coli and S. enterica (154). The chemosensory proteins that are activated by attractant or repellent molecules are stable, dimeric, transmembrane proteins called methyl-accepting chemotaxis proteins (MCPs). The periplasmic domains of different MCPs have little homology as they “sense” or bind different molecules. Within the cytoplasmic domain of MCPs is a highly conserved signaling domain where the adaptor protein CheW interacts with the cytoplasmic histidine kinase (HK) CheA (155). When the MCP binds a ligand, the signal is transmitted to CheA which, upon activation, phosphorylates the ▇▇ ▇▇▇▇ (156). Phosphorylated CheY (CheY~P) binds to the switch complex within the flagellar motor to promote CW rotation of the flagellum and tumbling occurs. CheY~P can be dephosphorylated by CheZ to terminate the signal (155, 157). The CW and CCW rotations of flagella exist in equilibrium with CCW rotation being favored unless CheY~P formation is initiated (154). Another method to reset signal transduction is through altering the methylation state of the MCP...