Azospirillium brasilense
Aaron Fairchild

 

In 1922, Bjernick was culturing for nitrogen-fixing soil bacteria when he happened upon colonies that seemed to stick around for only a little while, before being superseeded by other organisms. He found those particular gram-negative colonies could be induced to stay around longer when grown on a malate medium.   They appeared to him as spirillium under the microscope, but had metabolic attributes more similar to those of azobacter.   He therefore named his organism (genus name) Azospirillium.

In 1979 Tarrand, Krieg and Dobreiner sequenced the RNA of what was presumed to be a variety of Azospirillium lipoferum from Brazil.  It was found to belong to group of purple bacteria.   It was found to be different enough from A. lipoferum to warrant being given a different species name.   Hence Azospirillium brasilense.

The main physical difference between A. brasilense and A. lipoferum is that A. brasilense remains motile for a longer period of time after the colony has passed tropophase.   They are otherwise generally indistinguishable.

Despite the connotation of the name, this organism can be found almost anywhere.    Either A. brasilense or A. lipoferum are found in 30-90% of soil samples taken from around the world.  It is often found on the surface of plant roots, most notably on rice, wheat, barley, and oats.   It has been shown to interact with plants there: varieties have been found that supply fixed nitrogen, help in mineral uptake, and supply plant growth hormones.  Varieties have been found that do similar things within the root of the plant, and yet others that are pathogenic.

In aerobic conditions it produces a pink pigment, but it never photosynthesizes.  A. brasilense will not fix nitrogen aerobically: it is a microaerobic nitrogen-fixer.   In a semisolid nitrate-free medium, it's motility becomes apparent: it will make a thin layer below the surface to reach microaerobic conditions (flagellated polar and laterally).

A. brasilense grows best in when malate is present.   It produces exopolysaccharides in the absence of nitrogen, which have been suggested to have to do with its mutualistic properties.  Few of the plasmids associated with it's unique properties have been isolated.   Experiments continue to discern the secrets of this genre.

Interestingly enough, these imprortant microorganisms are less often found in agricultural soil.   It has been thought the compounds applied to heavily-used soil have thwarted their presence.

Flagella: Electron micrograph of Azospirillum brasilense ATCC 29145 cultured on MPSS agar at 30C for 24 H.   X15000

References

Journal of General Microbiology.   1988, v134, p2269-2279.

W. Luden, Gary P. Roberts.  Journal of Bacteriology.   May 1996 v178 p2948-2954.

Perombelon, M. -ed.  The Prokaryotes.  Second Edition. p2899- 2921

Bergey's Manual of Bacteriology, volume 1, p100-103.   1984


*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.

 

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