Myxococcus xanthus

Myxococcus xanthus is a Gram-negative bacterium commonly found in top soil. A voluntary multicellular microorganism, Myxoc occus xanthus distinguishes itself from other prokaryotes by virtue of its sophisticated social lifestyle.

When nutrients are sufficient, M. xanthus cells move as coordinated swarms. These swarms may contain thousands of cells, which secrete hydrolytic enzymes into their environment to lyse other cells and convert insoluble proteins into soluble, transportable amino acids. The feeding strategy of moving in swarms on the metabolizable substrate has been termed the "wolf pack" effect to reflect the fact that large numbers of organized cells undoubtedly utilize insoluble nutrients more efficiently than a single cell.

When the food supply runs low, M. xanthus initiates a complex developmental program, and tens of thousands of cells aggregate to form dome-like structures called fruiting bodies. Each fruiting body comprises ~10 5 cells and was constructed through coordinated movements. When the building movements complete, a portion of cells within the fruiting body differentiates into heat-resistant spores, which can re-germinate when the environments become favorable again (Fig. 1).

Fig. 1. Developmental cycle for M. xanthus.

Kaiser, D. (2003). "Coupling cell movement to multicellular development in myxobacteria." Nat Rev Microbiol 1(1): 45-54.

An important feature of M. xanthus behavior is its ability to move on a solid surface by a mechanism called "gliding". Gliding motility is a method of locomotion that enables bacterial movement on a solid surface without the aid of flagella. The study of gliding motility is important since it is widespread in nature and has been shown to be central for biofilm production and pathogenesis. Studies have indicated that M. xanthus gliding motility is regulated by two motility systems, the A (adventurous) and the S (social) motility systems. Whereas A motility is usually described as motility of well-isolated cells or small cell groups, S motility is described as coordinated movement of large cell groups.

S-motility was initially discovered as coordinated movement of large cell groups on 1.5% agar surfaces (Fig. 2). S-motility appears to be important for fruiting body formation, because many (but not all) known M. xanthus S-motility mutants are defective to various degrees in fruiting body development. Two M. xanthus cell surface appendages, type IV pili (TFP) and fibrils, are required for both S-motility and normal fruiting body development.

Fig. 2 Confocal image of myxococcus xanthus cell group

Type IV pili are polarly localized fibers measuring 5-7 nm in diameter and 4-10 mm in length (Fig. 3). Numerous experiments and reports have demonstrated the absolute requirement of the polar type IV pili for M. xanthus S-motility. For example, the removal of pili either by genetic mutation or by mechanical shearing leads to S-motility defects.

Fig.3 The Myxococcus xanthus Type IV pili, scale bar: 2μm.

M. xanthus fibrils consist of extracellular matrix material composed of approximately equal parts carbohydrate and protein surrounding the cells and linking neighboring cells to each other and to the substratum. Fibril-defective mutants are deficient in S-motility and fruiting body formation.

The myxo research in our lab focuses on understanding the molecular basis of M. xanthus Social motility. In the past few years, we’ve identified that TFP function as social motility motors via retraction, and the EPS matrix can trigger TFP retraction. We also identified the genetic locus responsible for EPS biogenesis. In addition, we’ve found that the Frz chemosensory system controls the TFP switching frequency via Frz phosphorylation. (see publication list)

The on-going projects include: 1) Understanding the molecular structure of PilA (pilin) and its interaction with EPS. 2). Understanding the modification of PilA in connection with the assembly/disassembly of TFP. 3) Elucidating the biochemical structure of EPS. 4) Analyzing the temporal production and spatial distributions of EPS as well as the eps gene regulation. 5) Understanding how the TFP-EPS-Frz interaction leads to chemotaxis at the cellular level.