Important Groups of Procaryotes (page 7)
(This chapter has 10 pages)
© Kenneth Todar, PhD
a unique resting cell called an endospore. They are
and usually rod-shaped, but there are exceptions. The two important
the members of which are aerobic
endospore-forming bacteria in the soils, and Clostridium,
whose species are anaerobic sporeformers of soils, sediments and the
tracts of animals.
Figure 16. Endospore-forming
bacilli (phase contrast illumination). Endospores are dehydrated,
cells appearing as points of bright light under phase microscopy.
bacteria are characterized by the location (position) of the endospore
in the mother cell (sporangium) before its release. The spore may be
terminal or subterminal, and the sporangium may or may not be swollen
accommodate the spore.
Figure 17. Anatomy of an
cross section drawing by Viake Haas. Endospores differ from the
cells that form them in a variety of ways. Several new surface layers
outside the core (cell) wall, including the cortex and spore coat. The
cytoplasm is dehydrated and contains only the cell genome and a few
and enzymes. The endospore is cryptobiotic (exhibits no signs of life)
and is remarkably resistant to environmental stress such as heat
acid, irradiation, chemicals and disinfectants. Some endospores have
dormant for 25 million years preserved in amber, only to be shaken back
into life when extricated and introduced into a favorable environment.
Figure 18. The sequential
in the process of endospore formation in Bacillus subtilis.
Some sporeformers are pathogens of animals, usually due to the
of powerful toxins. Bacillus anthracis causes anthrax,
a disease of domestic animals (cattle, sheep, etc.) which may be
to humans. Bacillus
cereus is becoming increasingly recognized as an agent of food
botulinum causes botulism
a form of food-poisoning, and Clostridium tetani causes tetanus.
Figure 19. Robert Koch's
photomicrographs of Bacillus anthracis. In 1876, Koch
by careful microscopy that the bacterium was always present in the
of animals that died of anthrax. He took a small amount of blood from
an animal and injected it into a healthy mouse, which subsequently
diseased and died. He took blood from that mouse and injected it into a
another healthy mouse. After repeating this several times he was able
recover the original anthrax organism from the dead mouse,
for the first time that a specific bacterium is the cause of a specific
disease. In so doing, he established Koch's Postulates, which still
supply the microbiological standard to demonstrate that a specific
is the cause of a specific disease.
In association with the process of sporulation, some Bacillus
species form a crystalline protein inclusion called parasporal
The protein crystal and the spore (actually the spore coat) are toxic
lepidopteran insects (certain moths and caterpillars) if ingested. The
crystals and spores of Bacillus thuringiensis are marketed as
a natural insecticide for use on garden or crop plants. Another species
of Bacillus, B. cereus, produces an antibiotic that
growth of Phytophthera, a fungus that attacks alfalfa seedling
roots causing a "damping off" disease. The bacteria, growing in
with the roots of the seedlings, can protect the plant from disease.
Also, apparently in association with the sporulation process, some
bacilli produce clinically-useful antibiotics. Bacillus
such as bacitracin and polymyxin are usually polypeptide molecules that
contain unusual amino acids.
Actinomycetes and related bacteria
are a large group of Gram-positive bacteria that usually grow by
formation, or at least show a tendency towards branching and filament
Many of the organisms can form resting structures called spores, but
are not the same as endospores. Branched forms superficially resemble
and are a striking example of convergent evolution of a procaryote and
a eucaryote together in the soil habitat. Actinomycetes such as Streptomyces
have a world-wide distribution in soils. They are important in aerobic
decomposition of organic compounds and have an important role in
and the carbon cycle. Products of their metabolism, called geosmins,
impart a characteristic earthy odor to soils. Actinomycetes are the
producers of antibiotics in industrial settings, being the source of
tetracyclines, macrolides (e.g. erythromycin), and aminoglycosides
streptomycin, gentamicin, etc.). Two bacteria in this diverse group are
important pathogens of humans:
Mycobacterium tuberculosis is the
cause of tuberculosis;
diphtheriae is the cause of diphtheria.
Also, many nonpathogenic mycobacteria and corynebacteria live in
Figure 20. Schematic diagrams
illustrating mycelial growth and spore formation in several genera of