Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Myxococcus xanthus predatory behavior in the presence of Escherichia coli. The black circle in the center of the photo is an aliquot of Myxococcus xanthus cells mixed with India ink that was pipetted adjacent to a long strip of E. coli cells covering the bottom half of the picture. As the M. xanthus swarm migrates into the E. coli colony, rippling behavior is induced to increase predation efficiency. Aggregates of cells form around the initial M. xanthus inoculum, with one lengthy aggregate forming amid the ripples. The aggregates eventually develop into mature fruiting bodies containing spores. (See related article on page 5888.)