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Abstract
Myxococcus xanthus, like other myxobacteria, is a social bacterium that moves and feeds cooperatively in predatory groups. On surfaces, rod-shaped vegetative cells move in search of the prey in a coordinated manner, forming dynamic multicellular groups referred to as swarms. Within the swarms, cells interact with one another and use two separate locomotion systems. Adventurous motility, which drives the movement of individual cells, is associated with the secretion of slime that forms trails at the leading edge of the swarms. It has been proposed that cellular traffic along these trails contributes to M. xanthus social behavior via stigmergic regulation. However, most of the cells travel in groups by using social motility, which is cell contact-dependent and requires a large number of individuals. Exopolysaccharides and the retraction of type IV pili at alternate poles of the cells are the engines associated with social motility. When the swarms encounter prey, the population of M. xanthus lyses and takes up nutrients from nearby cells. This cooperative and highly density-dependent feeding behavior has the advantage that the pool of hydrolytic enzymes and other secondary metabolites secreted by the entire group is shared by the community to optimize the use of the degradation products. This multicellular behavior is especially observed in the absence of nutrients. In this condition, M. xanthus swarms have the ability to organize the gliding movements of 1000s of rods, synchronizing rippling waves of oscillating cells, to form macroscopic fruiting bodies, with three subpopulations of cells showing division of labor. A small fraction of cells either develop into resistant myxospores or remain as peripheral rods, while the majority of cells die, probably to provide nutrients to allow aggregation and spore differentiation. Sporulation within multicellular fruiting bodies has the benefit of enabling survival in hostile environments, and increases germination and growth rates when cells encounter favorable conditions. Herein, we review how these social bacteria cooperate and review the main cell–cell signaling systems used for communication to maintain multicellularity.
Highlights
The existence of multicellular organisms in all the lineages of the tree of life suggests that multicellularity emerged on multiple occasions in the course of evolution (Rokas, 2008; Aravind et al, 2009)
A large fraction of these additional genes have appeared as a result of gene duplication events (Rokas, 2008). This is consistent with the expansion of myxobacterial genomes having arisen largely through gene duplications of specific gene families, those involved in cell signaling and signal transduction, which are likely to function in cell–cell interactions to maintain multicellularity and in response to changing environmental conditions (Goldman et al, 2006; Schneiker et al, 2007; Pérez et al, 2008; Huntley et al, 2011)
Many elements and contexts contribute to myxobacterial multicellularity
Summary
The existence of multicellular organisms in all the lineages of the tree of life suggests that multicellularity emerged on multiple occasions in the course of evolution (Rokas, 2008; Aravind et al, 2009). Is required for sustaining M. xanthus multicellularity, to maintain the integrity of cell groups, which is important for biofilm formation, cellular cohesion and connection of cells to the substrate, and because it participates in motility and fruiting body morphogenesis (Arnold and Shimkets, 1988a,b).
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