Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights Reserved.): Computer-generated animation of Escherichia coli chemotaxis. A cluster of receptors at one end of the cell (orange and yellow) generates a signal that changes the rotation of flagellar motors in the cell wall (light blue and purple). The signal is carried by the diffusion of a small protein CheY (black) in its phosphorylated form CheYp (red) (note the gradient of CheYp). (See related article on page 45.)