Geoscience Reference
In-Depth Information
flagella are important characters in the identification of bacteria.
Figure 42
shows two
photographs of motile flagellated bacteria.
F
IG. 42
Left
A salicyclic acid-decomposing
Azotobacter chroococcum
, showing typical flagella. (Photomicro-
graph by N. W., magnification ca. 1500).
Right
A nitrifying bacterium
Nitrosolobus
sp. showing typical peritrichous flagella, isolated from a Ghana
soil. (Flagella stain preparation by N. W. magnification ca. 1200.)
Another type of instrument, the scanning electron microscope is especially em-
ployed to observe the surface structure of specimens, after coating them with a thin
film of a heavy metal, usually gold. It is useful for viewing surface details of relat-
ively large organisms such as micro-arthropods as in
Figures 19-22
.
Bacteria will multiply in a wide variety of media, that is to say, solutions of suit-
able nutrients. By adding gelatin or agar to such media they are converted into a firm
jelly or so-called 'solid medium'. Bacteria can be inoculated on to the surface of such
solidified media and on keeping at appropriate temperatures they multiply and de-
velop into colonies of organisms which are generally circular in shape and may be
several millimetres in diameter. Colonies contain thousands of millions of individual
bacterial cells; the size, colour and texture of colonies are useful identifying features.
If the number of live bacteria seeded on to the surface of a solid medium is suitably
small, the number of colonies which develop after the appropriate incubation can be
readily counted. If we assume that each colony developed from one bacterium, then
the number of colonies corresponds to the number of bacteria inoculated initially on
to the medium. This is the plate culture or plating method for counting microorgan-
isms; it is also used to grow cultures of bacteria or other organisms. Moreover, it is
the method
par excellence
for the isolation of pure cultures as well as for detecting
contaminating microbes in presumed pure cultures, because different microorganisms
