Bacteria are the dominant life form on
Earth. There are more bacteria, as separate individuals, than any other
type of organism. In one teaspoon of fertile soil there can be as many
as 3 billion bacteria. Bacteria appeared on earth almost immediately
after it first cooled 4 billion years ago.
Bacteria can adapt to virtually any environmental condition. They are
found in the bodies of all living organisms and on all parts of the
planet -- in every land terrain and all ocean depths; in arctic ice
and glaciers; in boiling hot springs, and even in the stratosphere.
Bacteria species have been discovered that can only live deep beneath
the earth’s surface and others that thrive without sunlight in
the intense temperature and pressure near hydrothermal vents on the
ocean floor.
All bacteria are microscopic unicellular organisms. They are prokaryotic
- their cell nucleus and other organelles have no membranes. The DNA
is clustered unbounded in the cytoplasm. They come in different shapes:
round (coccus), rod-shaped (bacillus) or spiral (vibrio,
spirillum; spirochete)
It is estimated that 500 to 1000 different species of bacteria live
in the human body (Sears, 2005). Bacterial cells are much smaller than
human cells, and there are about ten times as many bacteria as human
cells in the body (100 trillion (1014) versus 10 trillion (1013); Sears,
2005). Though normal flora are found on all surfaces exposed to the
environment (on the skin and eyes, in the mouth, nose, small intestine,
and colon), the vast majority of bacteria live in the large intestine.
GRAM STAINING
This is a widely used microbiological technique that greatly aids in
the identification and characterization of bacteria. It was devised
by Danish physician, Hans Christian Gram, in 1884. The Gram reaction
reflects fundamental differences in the biochemical and structural properties
of bacteria. In the Gram stain procedure, all bacteria are stained purple
by the crystal violet stain. When a weak iodine solution is added, crystal
violet binds to the bacterial cell wall. Bacterial species that have
the ability to retain crystal violet after treatment with an organic
decoloriser such as acetone are called Gram-positive. In order to make
the Gram-negative cells visible, safranin is used as a counterstain.
Gram-positive organisms stain blue or purple while Gram-negative organisms
stain red or pink. Immediate examination of a Gram-stained smear of
material can often provide important data on which to base early clinical
decisions. In certain rapidly progressive infections the Gram-stained
smear may allow a presumptive etiologic diagnosis to be made much earlier.
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