Many prokaryotes are motile, the most obvious mechanism being "swimming" by rotating one (polar), two, or many (peritrichous) flagella. The flagellum consists of a long, helically shaped protein (flagellin) anchored in the cytoplasmic membrane and extending through the cell wall. Rotation of the flagellum leads to movement of the cell. Movement in the soil environment is important for unicellular organisms searching for new sources of nutrients. Flagellar motility is, however, unlikely to be useful for transport between pore size classes in soil as rates of movement are not great. Nevertheless, flagellar motility can be linked to chemotaxis (movement toward a chemical attractant), which appears to be important for communicating between bacterial cells and plant roots in the soil environment, e.g., in movement of rhizobia toward roots prior to nodule formation. Movement over long distances is, however, more likely to be achieved through bulk flow of water containing suspended cells or carriage on roots or soil animals. Indeed, lack of motility is probably important in the formation of microniches occupied by microcolonies of organisms, reducing competitive interactions, reducing selective sweeps, and increasing soil bacterial diversity (see Chap. 11). Other mechanisms exist for bacterial motility. For example, spirochetes possess an axial filament that enables the cell to move by flexing and spinning. Other organisms, including cyano-bacteria, Cytophaga, and mycobacteria, can move by gliding over surfaces.
metabolism and physiology
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