Myxococcus and other fruiting bacteria

The myxobacteria are an interesting family of gliding bacteria that produce fruiting bodies in starvation conditions. They are common in animal dung and organic-rich soils of neutral or alkaline pH. Some of them grow by utilising cellulose, but many of them feed by secreting antibiotics to kill other bacteria and then produce enzymes to lyse the cells of their prey. The genus Myxococcus is used as the example in the images below, because it is easy to grow in pure culture on agar plates.

The vegetative cells of all myxobacteria are aerobic, Gram-negative, elongated rods with either rounded or tapered ends (Figure D below). They glide in water films across solid surfaces, secreting slime (polysaccharide) tracks in which many cells migrate to produce feathery extensions at the colony margin (Figure A). At the onset of nutrient depletion the cells migrate back along the slime tracks, aggregating by chemotaxis, to form large concentrations of cells. These aggregates then develop into fruiting bodies (Figures B, C) which are raised above the agar surface and typically develop a bright yellow, red or brown pigmentation. As the vegetative cells migrate upwards into the fruiting body they undergo a progressive differentiation into rounded myxospores (Figure D).

B, C. Aggregation of vegetative cells to produce fruiting bodies just behind the colony margin shown in A. Figure B shows clearly that the cells are heaped on top of one another near the bases of the fruiting bodies; most of the slime tracks further away from these points contain few or no cells.

D. Part of a small fruiting body crushed by an overlying cover-clip. The elongated vegetative cells (bottom, left) were near the base of the fruiting body, the fully differentiated myxospores dark spherical bodies) were near the apex of the fruiting body, and cells in the centre of the fruiting body (top right) were in different stages of transition into myxospores.

Myxococcus species are easily cultivated on media containing a mixture of amino acids, which they use as the carbon and energy source; they do not utilise carbohydrates. Unlike some other myxobacteria, Myxococcus species produce simple fruiting bodies in which the stalk is composed of vegetative cells and slime. They also show a clear differentiation of the myxospores, which are spherical, refractile bodies. The myxospores are slightly heat-resistant, but not nearly as much as the endospores of Bacillus species. They resist UV irradiation and desiccation, surviving several years in dry conditions.

Colony of Myxococcus growing from a central streak on an agar plate containing amino acids. The edge of the colony is marked by arrowheads. Material from this colony was used to produce the images shown above. The mature fruiting bodies give the colony its faint pink colour.

Books and articles

M Dworkin and D Kaiser (eds) (1993) Myxobacteria II. American Society for Microbiology.

M Dworkin (1996) Recent advances in the social and developmental biology of the myxobacteria. Microbiological Reviews 60, 70-102.

Websites:

"The Myxobacteria Web page"

An interesting research site on myxob\cteria, with movies: ~kaiserla at cmgm.stanford.edu

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