Bacterial patterns

The delicately branching tree shown above (left) is formed by a complex multicellular assembly of the myxobacterium Chondromyces crocatus. A fruiting body of Stigmatella aurantiaca is shown on the lower right. Myxobacteria are gram-negative soil bacteria characterized by the type of social behaviour more often associated with eukaryotes. Their fruiting bodies contain resistant spores that arise as the culmination of a complex series of events set in motion by a changing nutritional and physical environment. Starvation causes myxobacterial cells to initiate the complicated signalling process that leads to spore formation. In Myxococcus xanthus, diffusible peptides are released and draw cells together. The swarming cells generate the striking pattern of ripples shown at the top right, as they migrate in equally spaced travelling ridges (the dark spots are fruiting bodies). It is not clear what induces the rippling, nor what holds the travelling ridges together. Once the swarming cells reach a critical density, cell-bound signals take over and morphogenesis of the fruiting body begins in earnest. Cells of S. aurantiaca forming an aggregation centre are shown in the centre-right panel. Unlike the familiar aggregation of the eukaryotic slime mould, Dictyostelium discoideum, in which individual cells are drawn together by a long-range diffusible signal, myxobacterial cells must be densely packed, and even organized into layers, to communicate with each other successfully and form a fruiting body. In M. xanthus aggregation mounds, the cells differentiate into inner and outer layers, with the inner cells less densely packed and forming the non-motile myxospores while the outer cells continue swarming. The pictures are reproduced (with permission) from a review of myxobacterial behaviour by