Domain Bacteria

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.