Figure 1. Patterns of swimming shown by different bacterial species. In all cases semi-rigid flagellar filaments are rotated by the transformation of the electrochemical ion gradient across the cytoplasmic membrane into mechanical work. Periods of rotation in one direction result in smooth swimming, interrupted by transient rotational switching. (a) E. coli swims smoothly by rotating its peritrichous flagella in a counterclockwise direction, which causes them to form a bundle that pushes the cell forward. Periodic switching to clockwise rotation causes the bundle to be pushed apart and the cell to tumble. The cell swims in a new direction when the bundle reforms. (b) Bacteria with a single polar flagellum change direction by transient reversals of the direction of rotation, being pulled rather than pushed. (c) The single flagellum of R. sphaeroides is only rotated in one direction. Transient stops result in transformation of the helical shape and reorientation of the cell when the functional helix reforms. (d) The internal filaments of spirochaetes drive the rotation of the helical cell through viscous media. The cell changes direction when the filaments rotate against each other and the cell flexes. In all cases the pattern of swimming results in random movement. Alteration of the direction changing frequency in a gradient results in biasing towards an optimum environment. (Adapted from Armitage JP (1992) Science Progress (Oxford) 76: 451–477.)

Figure 2. (a) Soft nutrient agar plates showing E. coli swimming down nutrient gradients formed by metabolic activity. The largest swarm is caused by wild-type bacteria swimming, the distinct rings resulting from different bacteria following gradients of different nutrients. The smaller swarm shows a mutant lacking one of the chemoreceptor for the major chemoattractant and therefore losing the outer swarm ring. The small dense colony is formed by a nonmotile strain. This clearly shows the advantage of being motile and chemotactic. (b) A very early photograph from Wilhelm Pfeffer showing bacteria accumulating around the mouth of a capillary containing a nutrient. (Adapted from Adler et al. (1969) Chemoreceptors in Bacteria Science 166:1588–1597. Reprinted with permission from AAAS. http://www.sciencemag.org.) (c) Chemosensory transduction in E. coli. MCP, methyl-accepting chemotaxis protein, the sensory receptor; W, CheW, the linker protein; A, CheA, the histidine protein kinase; P, phosphate added as a consequence of kinase activity; Y, CheY, the response regulator which binds to the motor switch to change the direction of rotation. B and R are CheB and CheR, the methyl esterase and transferase responsible for receptor adaptation; Z, CheZ, which causes signal termination by increasing the rate of CheY-P dephosphorylation; Aer, the MCP homologue, which senses the rate of respiratory electron transfer; E1–PTS, sugar transport system, which signals through CheA. Binding of an attractant to the periplasmic domain of an MCP causes CheA activity to decrease, resulting in a reduced concentration of CheY-P and thus smooth swimming. The reduction in CheB-P also results in a reduction in esterase activity and the increased relative transferase activity resets the MCP.