Myxobacteria: Moving, Killing, Feeding, and Surviving Together

The Supplementary Material for this article can be found online at: http://journal.frontiersin.org/article/10.3389/fmicb.2016.00781Myxococcus 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.This work has been funded by the Spanish Government (grants CSD2009-00006 and BFU2012-33248, 70% funded by FEDER) and Junta de Andalucía (group BIO318)

Integral proteins of the extracellular matrix fibrils of Myxococcus xanthus.

The extracellular matrix fibrils of Myxococcus xanthus are mediators of cell-cell cohesion and as such are required for the maintenance of the social lifestyle characteristic of these prokaryotes. The fibrils have also been implicated as factors involved in contact-mediated cell interactions and in signal exchange. The fibrils are extracellular carbohydrate structures with associated proteins. All of the major proteins associated with the fibrils react with monoclonal antibody 2105 and can be removed from the fibrils only by boiling with sodium dodecyl sulfate (SDS) and beta-mercaptoethanol. For consistency with their integral association with the fibrils, we have designated this class of proteins as integral fibrillar proteins class 1 (IFP-1). IFP-1 comprises five major proteins whose molecular sizes range from 66 to 14 kDa. All of the proteins in IFP-1 have been purified from isolated fibrils by electroelution after size separation on SDS-PAGE gels. Analysis of the purified proteins suggested that the forms with different molecular sizes result from the aggregation of a single small-molecular-size subunit. Fingerprint analysis and amino acid composition profiles confirmed the identity among the different members of IFP-1. The sequence of the 31 amino-terminal amino acids of the 31-kDa form of IFP-1 (IFP-1:31) was determined. There was no significant homology to other known protein sequences. During development there is a dramatic shift in the banding pattern of IFP-1 proteins without any apparent overall loss of total protein

Extracellular polysaccharides mediate pilus retraction during social motility of Myxococcus xanthus

Myxococcus xanthus is a Gram-negative bacterium with a complex life cycle that includes vegetative swarming and fruiting-body formation. Social (S)-motility (coordinated movement of large cell groups) requires both type IV pili and fibrils (extracellular matrix material consisting of polysaccharides and protein). Little is known about the role of this extracellular matrix, or fibril material, in pilus-dependent motility. In this study, mutants lacking fibril material and, therefore, S-motility were found to be hyperpiliated. We demonstrated that addition of fibril material resulted in pilus retraction and rescued this phenotype. The fibril material was further examined to determine the component(s) that were responsible for triggering pilus retraction. Protein-free fibril material was found to be highly active in correcting hyperpiliation. However, the amine sugars present in hydrolyzed fibril material, e.g., glucosamine and N-acetylglucosamine (GlcNAc) had no effect on fibril(−) mutants, but, interestingly, cause hyperpiliation in wild-type cells. In contrast, chitin, a natural GlcNAc polymer, was found to restore pilus retraction in hyperpiliated mutants, indicating that a polysaccharide containing amine sugars is likely required for pilus retraction. These data suggest that the interaction of type IV pili with amine-containing polysaccharides on cell and slime-trail surfaces may trigger pilus retraction, resulting in S-motility and slime-trailing behaviors