Myxococcus xanthus

Myxococcus xanthus exists as a self-organized, predatory, saprophytic, single-species biofilm called a swarm. It consists of rod shaped, gram-negative cells that exhibit self-organizing behavior as a response to environmental cues. The swarm, which has been likened to a "wolf-pack," modifies its environment via stigmergy. This behavior facilitates predatory feeding, as the concentration of extracellular digestive enzymes secreted by the bacteria increases. M. xanthus is a model organism for development; a behavior in which starving bacteria self-organize to form fruiting bodies: dome shaped structures of approximately 100,000 cells that, over the course of several days, differentiate into metabolically quiescent and environmentally resistant myxospores. During aggregation, dense ridges of cells move in traveling waves (ripples) that wax and wane over several hours.

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A swarm of M. xanthus is a distributed system; a population of superposable automata whose distribution is transparent so that it appears as one machine. It contains millions of cells that act as a collective, exhibiting coordinated movement through a series of signals to create dynamic patterns as a response to environmental cues. One of these behaviors, development (mentioned above), is controlled through a cascade of transcriptional regulators (TR) that control downstream gene expression. It has been proposed that all emergent behavior in M. xanthus is under this type of control.

Genomics

The complexity of the M. xanthus life cycle is reflected in its 9.14 MB chromosome, the largest prokaryotic genome sequenced until the sequencing of Sorangium cellulosum (12.3 Mb). This genome was recently sequenced by The Institute for Genomic Research (TIGR) and is currently available in GenBank.

Medical relevance

Beyond its use as a model for self-organization, M. xanthus also produces hundreds of potentially valuable secondary metabolites. During predation, M. xanthus releases over 300 secondary metabolites, many of which are used to lyse the cells of soil microbes on which it feeds. Many of these secondary metabolites are known to have medicinal properties. For example, M. xanthus produces myxalamid, an antibiotic that targets yeasts, molds, and enterobacteria. M. xanthus is also one of the most genetically tractable myxobacteria, and there is a considerable body of ongoing research aimed at genetically modifying M. xanthus to overproduce these compounds in a controlled environment, such as a fermentor. Current work includes the heterologous production of Epothilone B, a polyketide capable of targeting breast cancer cells that is superior to the anti-tumor drug Taxol. Finally, M. xanthus offers potential utility in the agricultural arena. Published data indicate that M. xanthus may act as a bacterial biological control agent to inhibit pathogenic fungi in plants.

Evolution

In 2003, two scientists, Velicer and Yu, deleted certain parts of the M. xanthus genome, making it unable to swarm effectively on soft agar. Individuals were cloned, and allowed to evolve. After a period of 64 weeks, two of the evolving populations had started to swarm outward almost as effectively as normal wild-type colonies. However, the patterns of the swarm were very different than those of the wildtype bacteria. This suggested that they had developed a new way of moving, and Velicer and Yu confirmed this by showing that the new populations had not regained the ability to make pili, which allows wild-type bacteria to swarm. This study provided more evidence for evolution, and it addressed questions about the evolution of cooperation between individuals that had plagued scientists for years.