Bacteria live on or in just about every material and environment on Earth from soil to water to air,
and from your house to arctic ice to volcanic vents. Each square centimeter of your skin averages about
100,000 bacteria. A single teaspoon of topsoil contains more than a billion (1,000,000,000) bacteria.
Bacteria consist of only a single cell, but don't let their small size and seeming simplicity fool you.
They're an amazingly complex and fascinating group of creatures. Bacteria have been found that can live
in temperatures above the boiling point and in cold that would freeze your blood. They "eat" everything
from sugar and starch to sunlight, sulfur and iron. There's even a species of bacteria-Deinococcus
radiodurans-that can withstand blasts of radiation 1,000 times greater than would kill a human being.
Bacteria [pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a
membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria
were eventually placed in a separate kingdom, Monera. Bacteria fall into one of two groups, Archaebacteria
(forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system
classifies the Archaebacteria, or archaea, and the Eubacteria as major groupings (sometimes called domains)
above the kingdom level. Bacteria were the only form of life on earth for 2 billion years. They were first
observed by Antony van Leeuwenhoek in the 17th cent.; bacteriology as an applied science began to develop
in the late 19th cent. as a result of research in medicine and in fermentation processes, especially by
Louis Pasteur and Robert Koch.
Bacteria are remarkably adaptable to diverse environmental conditions: they are found in the bodies of all
living organisms and on all parts of the earth-in land terrains and ocean depths, in arctic ice and glaciers,
in hot springs, and even in the stratosphere. There are more bacteria, as separate individuals, than any other
type of organism; there can be as many as 2.5 billion bacteria in one gram of fertile soil.
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