686 MICROBIOLOGY
environment is rather variable in that it can range from very
hot to very cold and from very wet to very dry, the heat- and
cold-resistant dormant spores offer the bacteria a means of
surviving adverse environmental conditions for months or
even years. Many important pathogens and commercially
important organisms are spore formers, e.g. Bacillus anthra-
cis which causes anthrax, Clostridium tetani which causes
tetanus and Clostridium acetobutylicum which can ferment
corn or potato mash into acetone, ethanol and butanol.
Corynebacteria are also rod-shaped bacteria but they
are pleomorphic and often look club-shaped. One of the
best known members of the genus is C. diphtheriae, which
causes diphtheria. Other members of the genus are commer-
cially important as producers of the vitamin folic acid.
Arthrobacter species are found widely in soil and water.
Depending upon the nutrients supplied, they can appear as
cocci or pleomorphic rods.4) Spirochetes are NOT true bacteria though they
resemble Eubacteriales in that they are spirally
curved, unicellular and multiply by binary fission.
They differ from eubacteria by the absence of
a rigid cell wall which allows them to be quite
flexible. They are all motile by means of axial
filaments attached at the cell poles and spirally
wrapped around the cell. The smallest spiro-
chete is 0.1 5 nm while the largest is 3.0
120 m. One of the most important spirochetes is
Treponema pallidum, which causes syphilis.
5) Actinomycetes are NOT true bacteria. Rather,
they are naturally-branching, filamentous, spore-
forming organisms which have a mycelial struc-
ture similar to that of filamentous fungi. Many
actinomycetes, especially those from the genus
Streptomyces, are commercially important sources
of antibiotics.
6) Mycobacteria are rods which can form a rudimen-
tary mycelium which resembles actinomycetes,
but they differ in that their cell walls are particu-
larly rich in waxes, which allows them to retainstain imparted by such dyes as basic fuchsin even
after treatment with dilute acid. This property,
called acid fastness, is characteristic of myco-
bacteria. Many species occur in soil but the best
known are the human pathogens M. tuberculosis
and M. leprae , which cause tuberculosis and lep-
rosy respectively.
7) Budding bacteria are NOT true bacteria. They
possess a complicated life cycle which includes
multiplication by budding rather than binary fis-
sion. Their type of budding can be readily dis-
tinguished from that of true fungi such as yeast.
The budding bacterium Hyphomicrobium exists
for part of its life cycle as a flagellated, slightly
curved rod. For multiplication, the flagellum is
lost, the chromosome replicates, and one chro-
mosome migrates to one end of the cell where
a hypha-like lengthening takes place. When the
hyphal extension ceases, it becomes a rounded
bud which contains the chromosome. The bud
grows in length and diameter until it reaches the
size of the mother cell, grows a new flagellum,
and separates from the hyphal extension.
8) Gliding bacteria are diverse group of prokaryotes
which are motile without having flagella. They
have very close affinities to blue-green algae
although gliding bacteria are not themselves pho-
tosynthetic. They may be unicellular rods, helical
or spiral-helical, or filamentous.
9) Blue-green algae or Cyanophyta are the only
prokaryotic algae. They are a diverse group that
include both unicellular and filamentous forms.
They have cell walls that resemble Gram-negative
bacteria but their photosynthesis more closely
resembles that of eukaryotes in that it is aerobic
rather than anaerobic (as in photosynthetic bacte-
ria). They are among the most complex prokary-
otes. Even though they lack defined organelles,
e.g. they lack chloroplasts, many species have
complex membranous or vesicular substructures
which are continuous with the cell membrane.Some fi lamentous forms contain specialized structures such
as gas vacuoles, heterocysts, or resting spores ( akinetes ).
Gas vacuoles are frequently found in planktonic species, i.e.
those which live in open water. These vacuoles are thought
to provide the algae with a means of fl oating and sinking
to the depth most appropriate to support photosynthesis.
Heterocysts arise from vegetative cells and are thought to
function in N 2 fi xation. Some blue-green algae show gliding
motility. None are fl agellated. They are very widely distrib-
uted either in terrestrial or aquatic habitats from the arctic
to the tropics. Some forms are found in hot springs. Other
Cyanophyta are symbionts in a variety of plants and animals.
For example a species of Anabaena fi xes N 2 for its host the
water fern, Azolla. Many blue-green algae form especially
luxuriant mats of growth called blooms which clog water-
ways and limit their use for navigation, etc.FIGURE 1 Animal viruses are often grown in embry-
onated eggs. The position of the hypodermic needles
indicates three common inoculation places.C013_004_r03.indd 686C013_004_r03.indd 686 11/18/2005 10:41:57 AM11/18/2005 10:41:57 AM