Myxobacteria: a model system for multicellular development and differentiation
Myxobacteria are Gram-negative soil bacteria that undergo a multicellular developmental program in response to nutrient deprivation. During this developmental program, approximately 100,000 cells coordinately construct a macroscopic fruiting body, and the vegetative cells differentiate into environmentally resistant and metabolically quiescent myxospores. Myxobacterial differentiation is initiated by nutrient limitation (e.g. carbon, nitrogen, or phosphate), similar to many differentiating microbes such as Bacillus, Streptomyces and Saccharomyces. The difference between the Myxobacteria and these other microbes is that control of the differentiation process for the individual cell is coupled to a multicellular developmental process: the formation of the fruiting body. Individual myxobacteria cells must evaluate both their own nutritional status, and of the nutrient status of the community. When nutrients are limited, the colonial associations of many other bacterial species cease and the cells disperse. In contrast, Myxobacteria colonies respond to starvation by constructing a multicellular structure, which facilitates colony survival. The survival advantage is that the fruiting body assures a localized concentration of spores, which, upon germination, quickly establishes a new microcolony. This provides Myxobacteria with an immediate competitive advantage in the soil environment. In nature, M. xanthus is bacteriolytic, utilizing the amino acids, peptides, and proteins of its prey. Because the cells can pool their secreted digestive enzymes, Myxobacteria colonies have a competitive advantage over isolated, individual cells. It has been postulated that this advantage is the evolutionary force favoring the close colonial associations observed.