Regulation of Multicellular Development in Myxobacteria

INTRODUCTION Water-insoluble organic matter constitutes the major source of nutrients in soil. Myxobacteria live in the soil where they feed on insoluble molecules such as cellulose, microbial cell walls, and the protein contents of bacteria and some yeasts. They degrade these materials with both secreted and cell-bound hydrolytic enzymes. A dense population is able to pool the action of digestive enzymes from many individual cells so that they feed like a pack of “microbial wolves” (Dworkin 1973). The efficiency of myxobacterial feeding on a protein substrate more than doubles with increasing cell density (Rosenberg et al. 1977). For this reason, apparently, myxobacteria have evolved ways to keeps their cells together in multicellular masses. Myxobacteria resemble higher organisms in that they move, feed, and lie dormant in multicellular groups. In fact, complexity of the fruiting bodies of some myxobacteria led to their mistaken identification as fungi for years (cf. Berkeley 1857) until Thaxter (1892) recognized their prokaryotic nature. Myxobacteria display their most impressive multicellular behavior when they are starved. Under these conditions, many thousands of cells move to aggregation centers where they mound on top of one another in specific ways that are characteristic for each of the five different families of myxobacteria (McCurdy 1974; Reichenbach and Dworkin 1981). This is illustrated for Myxococcus xanthus in Figures 1 and 2. Following aggregation, some cells within each mound lyse, whereas other cells differentiate into dormant, non-motile, thick-walled myxospores. Thus, the final structure, a mature fruiting body, has been formed by a primitive...