Mechanisms for cell interactions in Myxococcus xanthus

Myxococcus xanthus is a gram-negative soil bacterium that travels in a multicellular swarm to feed on other bacteria and soil detritus. Depletion of nutrients, presence on a solid surface, and high cell density serve as signals for many thousands of rod-shaped cells to aggregate into raised mounds and differentiate into spherical myxospores during the development of fruiting bodies. The purpose of this work was to study cell-cell interactions necessary for these social behaviors. The interactions were studied by genetic disruption through mutation and by use of an inhibitor which blocks those interactions.;The diazo dye Congo red specifically inhibited the same aspects of development as the dsp mutation by four independent assays. First, Congo red inhibited fruiting body formation by wild-type cells and reduced the yield of myxospores during submerged culture in starvation buffer. Dsp cells did not form fruiting bodies and produced few myxospores. Second, Congo red reduced the rate of wild-type gliding motility to a level comparable with that of Dsp cells in a swarm expansion assay. Third, wild-type cells which were treated with Congo red were noncohesive and did not agglutinate during the agglutination assay, whereas Dsp cells were also noncohesive and unable to agglutinate. Fourth, neither treated wild-type cells nor Dsp cells secreted an extracellular matrix of fibrils observed on untreated wild-type cells by electron microscopy. Removal of the dye from wild-type cells restored cohesion and production of the fibrils.;Binding studies of wild type and Dsp cells with Congo red showed that Dsp cells lack the major Congo red receptor present on wild-type cells. This receptor was extracted with a urea-Tris buffer and partially purified by equilibrium centrifugation and gel filtration. The material contains carbohydrate and protein.;Genetic analysis of the Dsp locus indicated that the region may contain multiple genes. Twenty-three DNA fragments were cloned from the wild-type genome near the Dsp locus. The wild-type phenotype was not restored to Dsp mutants by merodiploid construction. DNA hybridization by Southern analysis indicated that the fragments were derived from one orientation of the vector and may not have been long enough to encompass the entire Dsp region.