Microorganisms Study Guide

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Updated on Sep 20, 2011


Microorganisms (microbes) are very small, and most of them cannot be seen with the unaided eye, requiring the use of a microscope or at least a magnifying lens. We can also detect microorganisms by chemical tests. These living beings are everywhere, even in extreme environments such as very hot springs, very cold and dry areas, and even deep in the ocean under tremendous pressure. Some of these organisms cause diseases in animals, plants, and humans; however, most are beneficial to us and the Earth's ecosystems. In fact, we are utterly dependent upon microbes for our quality of life. This lesson will discuss three types of microorganisms: bacteria, protists, and fungi.


Bacteria are microorganisms that do not have a true nucleus; their genetic material is free floating within the cell. Bacteria are very small one-celled organisms and do not contain very complex cell structures. Generally, bacteria come in three varieties: bacilli (rod-shaped), cocci (sphere-shaped), and spirilla (spiral-shaped). Bacteria are prevalent in all environments and are important members of an ecosystem. They are responsible for the breakdown of dead organic matter into its constituent molecules. For this reason, we call bacteria decomposers. They also can be eaten by other organisms and are thus valuable in food-chain relationships. Since bacteria are small, can divide asexually very rapidly, can live practically anywhere, and have great metabolic versatility, they are the most numerous organisms on Earth. Many bacteria, when placed in good conditions, can reproduce every 20 or 30 minutes, each doubling its population after each reproduction.

Benefits of Bacteria

To illustrate the importance of bacteria, let's look at the cycling of the element nitrogen that is used by organisms to make proteins. We will start with dead plants that are being decomposed by bacteria. The nitrogen from the plant tissue is released into the atmosphere, and nitrifying bacteria converts that nitrogen into ammonia-type compounds. Other bacteria act upon these compounds to form nitrates that plants absorb. When these new plants die, we are back again at the decomposing bacteria that release the plant's nitrogen back into the atmosphere.

Bacteria are even in our intestinal tracts to aid in the digestion of food and the manufacturing of vitamins. We derive many benefits from bacteria, but they can also cause us to suffer with diseases.

Bacterial Diseases

Microorganisms, including bacteria, cause many diseases. These organisms enter our bodies in a variety of ways, including airborne transmission, ingestion by mouth, or through the skin when it is cut or injured. We can eliminate this threat by disinfecting utensils and hands or even by the sterilization of objects (the application of high-pressure steam heat). All these methods destroy bacteria and other microorganisms that may cause disease.


This group is composed of single-celled organisms that have their genetic material contained within a nucleus and have some specialized structures within their cells. These organisms are considered to be more primitive than other organisms with cellular nuclei, but they are more evolved than bacteria (Kingdom Monera). This is a diverse Kingdom consisting of organisms with varied structures and functions, such as amoeba and paramecium. Some of this Kingdom's members are autotrophic and contain chlorophyll, whereas others are heterotrophic and must eat other organisms. It is believed that early protists were both animal- and plant-like because they were able to obtain food in both ways. Today, a protist called Euglena does this. Protists are important parts of food chains and ecosystems, and some protists cause disease.

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