Bacteria have been found many miles high in the atmosphere, at the bottom of rivers and lakes, and in the deepest mines. Some are able to flourish at temperatures high enough or low enough to kill other forms of life. Most bacteria are smaller than yeasts, but all are larger than viruses. A unit of length called a micrometer, or micron, is used to measure bacteria. It takes 1,000 microns to make up one millimeter and about 25,000 microns to make up one inch.
Some bacteria are so small that their largest dimension is only one half a micron.
Most bacteria range from 1 to 5 microns in size. The electron microscope, which can magnify more than 300,000 times, has greatly increased man's knowledge of bacteria. The compound microscope, the only one available for many years, was limited to a magnification of about 1,000. How Bacteria Live Bacteria absorb their food and secrete their wastes through the cell wall. They secrete enzymes that break down the food into soluble form so that it can pass through the wall.