Microbiology - Microorganisms
3.1 Bacteria (singular: bacterium)
The bacteria are not at all simple, but appear less complex than other microorganisms.
They are relatively small. The diameter is typically about 1 - 2 mm, (but
can be as little as about 0.1 mm) and if they are not spherical, the length
may be about 2 - 10 mm. The spherical ones are called cocci (singular:
coccus). Rods are bacilli (singular bacillus) (but
remember that Bacillus is also a genus). A rod with some spiral form is
a spirillum (while spirochetes are much smaller and flexible).
Bacteria increase in numbers by dividing, approximately in half. This can
happen in a very short time C as little as 15 minutes C which means that
it is possible to grow many generations of bacteria from one day to the
When bacterial cultures are grown, there may be very little growth for 3
- 4 hours after inoculation. This period is called the lag phase.
Then growth increases rapidly, and a graph of the logarithm of the number
of living bacteria versus time becomes a straight line. This is called the
log phase (because of the logarithmic relation). Eventually the culture
becomes crowded and nutrients are used up, and the growth rate decreases
to zero in the stationary phase. That is followed by a death phase
as most of the bacteria die. The generation time, or doubling
time, is the average time between divisions.
The major structures in bacteria are:
||A relatively rigid outer layer which supports the cell
membrane against osmotic pressure. Some bacteria have a more or less
thick capsule, usually polysaccharide, outside the cell
|A complex structure just inside the cell wall. It is
a barrier against diffusion, and many important enzyme systems are
structurally part of the membrane. Because it is a diffusion barrier,
osmotic pressure develops across the membrane, which breaks unless
it is supported by the cell wall.
||Everything inside the cell membrane, including nuclear
material and many enzymes.
||The name was initially applied to structures in plant
and animal cells because they stained intensely with certain dyes,
but the bacterial chromosome is, at best, very difficult to visualize
(there is no separate nucleus). There is only one chromosome (i. e.,
bacteria are haploid), and it is typically circular.
||In some species, the cell wall can be dissolved by enzymes
(notably lysozyme). That leaves the thin flexible cell membrane holding
the cytoplasm. Because the cytoplasm is a concentrated solution, water
diffuses inward and the protoplast swells and bursts unless the enzyme
treatment is done in a solution with high osmotic pressure, such as
20% sucrose. If the protoplast does burst, it leaves a solution of
the cytoplasm and a suspension of small structures such as ribosomes,
and the empty membrane, which has various names, including ghost.
Many of the enzymes important for the cell biochemistry are built
into the membrane.
|Some bacteria have these tiny filaments which propel
them through liquids. They appear to be spiral structures which are
spun, like propellers, by little power units inside the cell. They
may be monotrichous (one filament at one end of the cell), amphitrichous
(one or more filaments at both ends), lophotrichous (multiple filaments
at the ends) or peritrichous (filaments all around the cell).
|Pilus (plural pili)
||Some bacteria can produce short tubes of protein through
which nuclear material can pass between cells (conjugation). They
also function to attach bacteria to surfaces. Similar but shorter
tubes are fimbriae (singular: fimbria).
||Some species can produce spores (endospores) which are
extremely resistant to drying, heat, and other hostile conditions.
Some have been reported to have survived for 1300 years. This is a
survival mechanism, not reproduction, as one cell becomes one spore,
with no surviving cell. (Molds can produce spores for reproduction;
they are not resistant.)
||relatively short lengths of nucleic acid, in circles
or loops, which are found outside the chromosome, but which can also
become part of the chromosome. They are very important in genetics
and genetic engineering. They are designated by code names such as
pBR322 which obviously cannot be translated.
The various bacterial species are capable of a very wide
range of activities. One can convert solid sulfur into sulfuric acid (up
to about 0.1 N concentration, about pH 1). Another can use cyanide (CN-)
as its sole source of carbon, nitrogen, and energy. Still another can
grow on substances remaining in good-quality distilled water.
Some bacteria grow well in the presence of air (oxygen). Those are aerobes.
Some require oxygen, and are obligate aerobes. Others cannot tolerate
oxygen, and are anaerobic. Some facultative organisms can
grow with or without oxygen. Still others tolerate oxygen, but grow much
better at oxygen concentrations much lower than the concentration in air.
Those are microaerophilic. Then there are temperature relations:
psychrophiles grow best (though still relatively slowly) at refrigerator
temperatures. Mesophiles grow at ordinary temperatures (about
20 - 40 °C). Thermophiles grow well only at high temperatures (65
- 100 °C).
Heterotrophic bacteria require relatively complex materials for
growth, while the autotrophic bacteria produce their own complex
materials from very simple sources. The organisms which can grow on cyanide
as their only source of carbon, nitrogen, and energy are autotrophs. Organisms
which require only inorganic materials may be called lithoautotrophs.
One rather vague term, eubacteriales, indicates true bacteria,
as distinguished from something else. At present, the primary something
else is represented by the archaeobacteria (or Archaebacteria).
Those are found most commonly near ocean thermal vents, at depths about
1500 meters with very high pressures and temperatures up to 250°C. They
are still very poorly known. Because of their growth conditions, they
Like all the organisms other than bacteria, yeasts are eucaryotes
(they have nuclei separated inside the nuclear membrane) and have pairs
of chromosomes (they are diploid). The cells also contain mitochondria
and other organelles. The mitochondria are small structures primarily
important for producing most of the energy used by cells. They have their
own nuclear materials and reproduce independently of the cells which contain
them. They strongly resemble bacterial protoplasts, and may have developed
from them. Organelles are various subcellular structures with specific
Yeasts reproduce by fission, but by budding rather than by dividing
in half. Some species of yeast have been used since ancient times in brewing
and baking. Some have been grown for food, but that is not (yet) a major
industry. When grown for food, yeast cultures are supplied with abundant
air, and they convert their energy source (typically glucose) to carbon
dioxide and water. In bread dough, the carbon dioxide produces bubbles
which make the bread rise. In the absence of oxygen, yeast grows less
well because it can convert glucose only to ethyl alcohol. That provides
much less energy than the complete oxidation of alcohol to carbon dioxide.
While alcohol production is fermentation, the same term is often
applied to any anaerobic culture, and sometimes to microbial culture in
These are rather like yeasts biochemically, but they divide more like
bacteria, usually with the rod-shaped cells remaining connected to form
filaments (mycelia; singular: mycelium). They also produce
reproductive spores which are not particularly resistant. Molds are strictly
(obligately) aerobic. They were the original source, and remain a major
source, of antibiotics. One mold, Neurospora crassa, was a major
source of information about genetics.
3.4 Algae (singular: alga)
These are microscopic plants (although some large seaweeds are also considered
to be algae). They contain chlorophyll and can carry out photosynthesis,
using the energy from light for their metabolism rather than getting all
their energy by oxidizing glucose (which they do in the dark). Most algae
perform photosynthesis in organelles called chloroplasts which
might have originated as simpler algae trapped within other cells. The
blue-green algae lack the separate chloroplast structure, and contain
not only chlorophyll but also a water-soluble blue phycocyanin.
Large-scale culture of algae has generally been limited to applied research
on closed ecological systems for space travel, in which the algae were
to convert the carbon dioxide exhaled by the crew back to oxygen, using
light for energy. That work, mostly done in the late 1950s and 1960s failed
partly because of the difficulty of providing light efficiently, and largely
because the crew would have had to eat all the algae. More recently, algae,
especially the blue-green Spirulina, have been grown profitably
as food supplements.
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Copyright © 2010 Denzel Dyer, all rights reserved.