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MICROORGANISMS. Microorganisms are organisms (forms of life) requiring magnification to see and resolve their structures. "Microorganism" is a general term that becomes more understandable if it is divided into its principal types—bacteria, yeasts, molds, protozoa, algae, and rickettsia—predominantly unicellular microbes. Viruses are also included, although they cannot live or reproduce on their own. They are particles, not cells; they consist of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), but not both. Viruses invade living cells—bacteria, algae, fungi, protozoa, plants, and animals (including humans)—and use their hosts' metabolic and genetic machinery to produce thousands of new virus particles. Some viruses can transform normal cells to cancer cells. Rickettsias and chlamydiae are very small cells that can grow and multiply only inside other living cells. Although bacteria, actinomycetes, yeasts, and molds are cells that must be magnified in order to see them, when cultured on solid media that allow their growth and multiplication, they form visible colonies consisting of millions of cells.
Many people think of microorganisms mainly in terms of "germs" causing diseases, but some "germs" are beneficial to humans and the environment. Diseasecausing (pathogenic) microorganisms need to be controlled, and in many cases, beneficial microorganisms are also controlled in plant and food production.
For thousands of years, people had no concept or knowledge of organisms invisible to the naked eye. In fact, it is only within the last several hundred years that magnification systems (lenses, magnifiers, microscopes) were developed that enabled scientists to observe microorganisms. In 1673 Antoni van Leeuwenhoek, a linen merchant in Delft in the Netherlands, was the first to observe and study microorganisms, using single lenses that magnified objects fifty to three hundred times. The role played by microorganisms was not clarified until the 1830s, when Theodor Schwann in Germany demonstrated that yeasts were responsible for alcohol production in beer and wine fermentations.
In 1854, Louis Pasteur in France found that spoilage of wines was due to microorganisms (bacteria) that convert sugars to lactic acid, rather than the alcohol produced by yeasts. He developed the process of "pasteurization," in which the temperature of food materials is raised to about 140 to 158°F (60 to 70°C), thereby killing many spoilage organisms. Pasteur also discovered that certain bacteria are responsible for the souring of milk. Today, milk is generally pasteurized to reduce its content of microorganisms, to extend its keeping quality, and to protect against pathogenic microorganisms that may be present.
Pasteur also discovered that each type of fermentation, as defined by the end products, is caused by specific microorganisms and requires certain conditions of acidity or alkalinity. He discovered further that some microorganisms, the aerobes, require oxygen and others, the anaerobes, grow only in the absence of oxygen. The latter probably developed in the earliest days of the earth when there was no oxygen in the atmosphere.
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A microorganism or microbe is an organism which is too small to be seen by the unaided human eye. They are microscopic. The study of microorganisms is called microbiology. Microorganisms include bacteria, fungi, archaea or protists and viruses. The term microorganisms does not include viruses and prions, which are generally classified as non-living.
