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:::BACTERIA::: Bacteria
are an abundant, widespread and prolific (reproduce in great quantities)
family of organisms belonging to kingdom Monera. They live
everywhere - in air, water, soil, milk, and even in the human body.
Bacteria are commonly despised because they are best known as causers of
disease. However, there are many which are directly and indirectly
useful to man, and some are actually necessary for life's activities.
A typical bacterial cell has a cell wall made of cellulose or
other material, surrounded by a thin, slimy gelatinous capsule. They
have no nuclei; instead, their chromatin granules are
often clumped together to resemble one large chromosome.
Bacteria are very small; usually less than 1/25,000 of an inch, a unit
of measurement called a micron. They vary from 0.15 to 5 microns
in length. Fifteen hundred of the rod-shaped bacteria hardly reach
across the head of a pin. Since they are visible under the microscope,
bacteria have been seen to have various and distinct shapes. Some have no powers of
locomotion, and are carried from place to place by the air, by water and
other liquids, and by plants and animals. Others have flagella with
which they move about in a liquid medium.
The surface soil, especially if it is rich in organic matter, contains
many kinds of bacteria; but at a depth of ten feet they usually
become scarce. Ordinary earth may yield from 10,000 to 100,000,000 in a
gram if it has become polluted. Soils that have not been cultivated have
fewer.
The condition that really determines how many bacteria will be found in
the soil is the amount of organic matter it contains.
The bacterium in soil breaks down parts of dead plants and
animals, facilitating soil enrichment. This is an important process
that is necessary for plant growth and survival.
CONDITIONS OF GROWTH
Like all other living organisms, bacteria must have all the proper
conditions before they can grow and multiply. Their food is chiefly plant
or animal matter. However, they cannot make use of food except in the
presence of warmth and moisture. Most bacteria require oxygen in addition,
which they get from the surrounding air.
When food, air, warmth, or moisture is not sufficient, bacteria cease to
grow and go into a resting state (encystment). That is, they change
their form, and surround themselves with a substance that protects the
soft protoplasm from being harmed by freezing, heating, or drying.
Bacteria, being living cells, must carry on all life processes.
Characteristically, they have a cell wall that surrounds a cell
membrane. Under favorable conditions, they reproduce by binary fission
(splitting in half after maximum growth). They get their food from other
plants and animals.
Conditions favorable to growth and reproduction of bacteria are:
1. Moisture
2. Oxygen usually from air
3. Moderate heat
4. Lack of direct sunlight
5. Prepared food
6. Proper chemical environment
Unfavorable conditions for growth and reproduction are, then:
1. Dryness
2. Oxygen
3. Extreme heat or cold
4. Direct sunlight
5. Lack of food
There are some hardy forms of bacteria that adapt themselves to
withstand unfavorable conditions, sometimes temporarily and
sometimes for many years. Their cell walls become very hard,
developing into spore cases or cysts. The cell within the spore
case is quiescent until conditions
are favorable again. In some measure, it resembles a bear, hibernating
during the long, cold, unfavorable winter.
APPEARANCE OF BACTERIA
Bacteria are grouped into three classes according to their shape:
(1) round (the cocci)
(2) rod-shaped (the bacilli)
(3) those that are shaped like a cork
(4) screw (the spirilla).
The spirilla and the bacilli often have on one or both ends tiny
thread-like hairs called flagella by which they move.
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Types of bacteria
Rickettsiae are like bacteria in that they reproduce by fission.
They are like viruses in their need for living tissue in order to
grow. The diseases caused by rickettsiae, including typhus fever,
are carried by insects and other animals. |
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