Bacteria are the smallest living things with a cellular structure; each individual bacterium consisting of one single colorless cell, which is usually either spherical or rod-shaped. Individual bacteria measure from 0.0001 inches to 0.00001 inches in length, so they can be seen only with the help of a high-power microscope. They are so small that they can float in the atmosphere, usually as 'passengers' on dust particles, up to a height of several thousand feet, except immediately after a heavy downpour, when the air is washed clean.
Bacteria are present in all natural as well as in drinking water that has not been purified or bailed. A large number of bacteria live in the soil, down to a depth of several feet, and they are particularly abundant in faces and sewage. Thus, living bacteria are always present on the surface of our bodies and on everything around us, but they are seldom found inside the tissues of healthy plants and animals.
Bacteria reproduce by dividing into two, and these new individuals grow so quickly that they are ready to divide again in about half an hour. Hence, in ten hours, under the most favorable conditions, a single bacterium can produce over a million bacteria. That is one reason for it being so difficult to ensure any object is completely free from any kind of living organisms. In addition, some forms of bacteria have a waxy envelope outside their cell wall and are thus more difficult to kill.
Few bacteria can long survive a temperature above 80°C in the presence of moisture. Hence, when food items are boiled, nearly all the bacteria present is killed. Pasteurization is a milder heat treatment that destroys the bacteria in milk. The rate of multiplication of bacteria is greatly slowed down at temperatures below 10°C. This means that food will remain unaffected by bacteria in a refrigerator.
Drying is also another method of preserving food and this dehydration of foodstuff prevents bacteria from growing and multiplying as there is insufficient moisture.