Myxobacteria

Abstract

Myxobacteria are Gram‐negative, rod‐shaped bacteria that are nearly ubiquitous in the biosphere. They forage on living and dead decaying material including bacteria and eukaryotic microbes. As such, myxobacteria appear to play an important role as scavengers cleaning up biological detritus in the environment. Isolates belonging to different genera can be distinguished by the shape of their vegetative cells, motility, pigments, as well as by the type of fruiting bodies and spores produced during their developmental cycle, which is a unique feature of this group. The ability to manipulate myxobacteria genetically combined with their complex processes including synthesis of secondary metabolites, foraging and predation, TraA‐dependent kin‐recognition, outer membrane exchange, fruiting body development, phase variation and gliding motility – makes them excellent research subjects.

Key Concepts

  • Social interactions in the myxobacteria are important for swarming, feeding and development.
  • Kin recognition and sibling killing involve a cell surface receptor, TraA.
  • Phase variation yields cells with specialised functions.
  • Predation involves release of hydrolytic enzymes that allow myxobacteria to feed on other microbes.
  • Myxococcus cells use small molecules (signals) to communicate and coordinate the fruiting body construction.

Keywords: development; sporulation; phase variation; gliding motility; secondary metabolites

Figure 1. Myxobacterial fruiting body images. Each fruiting body is a few tenths of a millimeter tall. (a) Myxococcus fulvus, (b) Stigmatella aurantiaca, (c) Chondromyces crocatus, (d) close up of a Chondromyces fruiting body showing the large number of rod‐shaped cells inside, (e) yellow fruiting bodies on rabbit dung, (f) fruiting bodies of Myxococcus xanthus. (c, d) Provided by George Barron, University of Guelph.
Figure 2. Gliding motility in M. xanthus. (a) Fully motile myxobacteria cells spread on vegetative medium to produce large colonies that are a few cell layers thick. (b) Individual cells (dark lines and clusters) and ‘slime’ trails (light colored lines) can be seen at the edge of a colony. (c) Cartoon showing models for the two forms of gliding motility in M. xanthus. T4P gliding (S‐gliding) requires the membrane proteins, which include PilB, PilT, PilQ and Tgl, shown in brown and black at the cell pole. They facilitate polymerization of PilA subunits for extension (arrows 1 and 2) and retraction (arrow 3) of the type IVa pili that extend from the pole of the cell on the left. When a pilus makes contact with exopolysaccharide on an adjacent cell, it contracts, dragging the cell towards the anchor point. The Dif and Eps proteins are needed for biosynthesis as well as for the export of polysaccharide. Cells also can glide (A‐gliding) using the Agl membrane protein complexes that generate movement as they migrate towards the lagging end of the cell or as the cell rotates along the helical cytoskeleton. The T4P and Agl gliding systems can operate independently and are coordinated by MglA, MglB, Rom and Frz proteins.
Figure 3. Predation gives M. xanthus cells a mechanism to feed off other microbes. This image illustrates the effects of predation after several days of co‐incubation with M. xanthus (left) and Escherichia coli (right) on agar. Initially, aliquots of cells were placed next to each other on the agar. As M. xanthus cells began to swarm (colonial gliding behaviour), destruction of the E. coli cells became apparent by the clearing in the middle of the image (overlap area). M. xanthus cells respond by rippling and forming mounds (visible on the left part of the image) (Berleman et al., ). Reprinted with permission from PNAS © (2008) National Academy of Sciences, USA.
Figure 4. Life cycle of M. xanthus. When nutrients are available, cells divide by binary fission (left panel) and forage in the environment. Starvation (dashed vertical line) will induce cells to enter the part of the life cycle that involves production of a spore‐filled fruiting body. Nutrient‐poor conditions elicit a unified starvation stress‐response that initiates a complex programme of self‐organisation, which changes cell behaviour and leads to the formation of densely packed aggregates. These complex behaviours include wave formation (rippling), streaming into aggregates, mound building, and finally sporulation. Timeline: at 0–6 h, cells aggregate (top and side views) and activate signalling pathways described in the text (molecular view). By about 12–16 h, patterns called rippling are visible as cells align along their long axes and reverse direction upon pole‐to‐pole contact. Aggregates of cells coalesce to form tight mounds by 18 h. At 24 h the fruiting body is a spherical structure of approximately 105 cells that contains stress‐resistant myxospores. The fruiting body is sticky and its spores are tightly packed. When a fruiting body receives nutrients, the individual spores germinate (left panel), elongate to rod‐shaped cells, and begin to divide as vegetative cells.
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Further Reading

Berleman JE and Kirby JR (2009) Deciphering the hunting strategy of a bacterial wolfpack. FEMS Microbiology Reviews 33: 942–957. Bacteria on the move: NYT permalink http://nyti.ms/25TOpo9.

Bulyha I et al. (2011) GTPases in bacterial cell polarity and signaling. Current Opinion in Microbiology 14 (6): 726–733.

Dey A, Vassallo CN, Conklin AC, et al. (2016) Sibling rivalry in Myxococcus xanthus is mediated by Kin recognition and a polyploid prophage. Journal of Bacteriology 198: 994–1004.

Wall D (2014) Molecular recognition in myxobacterial outer membrane exchange: functional, social and evolutionary implications. Molecular Microbiology 91: 209–220.

Westra ER et al (2014) CRISPR–Cas systems: beyond adaptive immunity. Nature Reviews. Microbiology 12: 317–326.

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How to Cite close
Hartzell, Trish(Aug 2016) Myxobacteria. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020391.pub2]