BIOL/CSES 4684

BRADYRHIZOBIUM

SOIL MICROBIOLOGY

This webpage was created by Adrienne Harris

1. IDENTIFYING CHARACTERISTICS

Aerobic.
Rods.
0.5-0.9 um x 1.2-3.0 um in size.
Non-sporeforming.
Cells stain Gram negative.
Motile.
Chemoorganotrophic.
Cellulose and starch not utilized.
Casein and agar are not hydrolyzed.
Ammonium salts and some amino acids serve as nitrogen sources.
The percent G+C ranges from 61 to 65%.
Colonies are white and opaque.
Slow-growing on artificial media.

The above picture is an electron micrograph of B. japonicum cells.

The above pictures show a soybean nodule containing B. japonicum that has been cut open (left), and a nodulated soybean plant (right).

2. TAXONOMIC DESCRIPTION
The "bradyrhizobia" are an extremely heterogeneous group of nodule bacteria in which the taxonomic relationships are not well understood. Motility occurs by one polar or subpolar flagellum. Metabolism is aerobic and respiratory with oxygen as the terminal electron acceptor. The maximum growth temperature ranges from 30 to 42 degrees Celsius, but most strains do not grow above 39 degrees Celsius. Strains are usually acid tolerant, though not exclusively, and grow at pH 4.5. Growth usually does not occur above pH 9.0.

Slow-growing Rhizobium bacteria are very different from the bacteria of the fast-growing group, so they were placed in a separate genus, Bradyrhizobium. Currently, three species, B. japonicum (soybean), B. lupinus (lupin), and B. arachis (peanut), have been classified. Several other species are simply called Bradyrhizobium spp.

3. ISOLATION AND ECOLOGY
Isolation is done by the same techniques used for Rhizobium. An alkaline reaction is produced in mineral salts medium containing mannitol or other carbohydrates. Growth on carbohydrate media is usually accompanied by extracellular polysaccharide slime. Bradyrhizobium colonies tend to be granular in texture. They are circular, opaque, white, and convex. Bradyrhizobia cause nodule production on soybean, cowpea, siratro, and a wide variety of leguminous plants. A highly specific nodulation of the nonleguminous plant Parasponia occurs. It is the only validated instance of nodulation by a Rhizobium or Bradyrhizobium species occurring outside of the leguminous plants.

The picture to the left shows a streak plate from a soybean nodule crushed on mannitol-salt agar.

4. ADDITIONAL SOURCES OF INFORMATION
Broughton, W.J., Puhler, S. 1986. Nitrogen Fixation Volume 4: Molecular Biology. Oxford University Press. Oxford.

Gresshoff, P.M. 1990. Molecular Biology of Symbiotic Nitrogen Fixation. CRC Press, Inc. Boca Raton, FL.

Krieg, N. 1984. Bergey's Manual of Systematic Bacteriology. Williams and Wilkins. Maryland. Sprent, J.I. and P. 1990. Nitrogen Fixing Organisms: Pure and Applied Aspects. Chapman and Hall. New York, NY.

5. LINKS TO OTHER SITES ON BRADYRHIZOBIUM
BIONET Nitrogen fixation in plants: legumes, nodules and rhizobium.