BACTERIA AND OTHER MICROORGANISMS OF THE SOIL - The Soil - page 126

F IG. 43
Above Electron microscope photograph of cells of a tiny nitrifying bacterium isolated from a mountain
soil (Snowdonia). This recently recognized Nitrosovibrio sp. is a small vibrio. (Phosphotungstate negative
stain by N.W., magnification ca. 20,000).
Below Photograph of colonies of a Nitrobacter sp., a nitrite-oxidizing bacterium isolated from an Argen-
tinian soil. The circular colonies were grown on the surface of nitrite/agar medium (magnification x 8.)
Many bacteria are surrounded by a capsule which serves as a protective coat and
contains a kind of mucilage composed of different polysaccharides or other organic
compounds. This mucilage, incidentally, causes organisms to adhere together as well
as helping to cement soil particles into more stable soil crumbs, thus contributing to a
useful soil structure. The capsular material is also an important factor which determ-
ines the specificity of the immunological properties of the microorganism.
The formation of spores or resting forms is another feature of many species of
bacilli and bacteria. A spore is a resting stage which can survive adverse conditions,
especially desiccation. When favourable growth conditions again prevail, then the
spore can germinate and become a growing vegetative cell once more. Spores are
usually spherical or ovoid in shape, often larger in diameter than the rod-like cell
and so they assume a drumstick-like appearance under the microscope. Bacillus and
Clostridium are the names of two genera of spore-forming bacteria the former be-