Bacteria: Friends and Foes
By Harold Eddleman, Ph. D.

We look upon bacteria which kill humans as being harmful. That same bacterium may have made our present civilization possible by killing off over populations of the past which would otherwise destroyed our forests and other resources so that we could not destroy the resources now to build our homes and universities. The bacterium that lives in the gut of the termite enabling him to eat the wood frame of our house, enables the same termite to eat (on a different day) the wood of a tree stump in the yard to promote its decay and enrichment of a flower bed.

While true, the first example above is not very pleasant to think about. Therefore, we have various foundations to collect money to fight bacteria diseases cheaply and quickly so we save money which we can use to buy the trees and resources of other countries so the animals and humans of those countries perish from lack of food and resources. In these examples, economics has enabled additional types of competition between species for available resources.


play a role in making the workStart with this page to learn about bacteria. At the bottom of this page you will find links to other pages where you can learn more about bacteria. I have written these pages to help beginners get started and to help advanced students find information they need. Please help by sending me an e-message containing your ideas for improvement.

Lets begin our study of bacteria by considering the balance of nature. If there is food, some organism will eat it. If there is a place to live, some organism will live there. Every species has a great ability to produce offspring and its population expands until it runs out of food or it is limited by competition, its own waste products, or some other factor. Changes in climate or introduction of a new species from elsewhere can greatly affect the balance of nature. These simple sentences summarize the interactions of living things on earth.

Bacteria are single-cell organisms and most of them must find foods such as sugars, proteins and vitamins to live. The blue-green bacteria (some times called blue-green algae) have chlorophyll and can make their own food from light energy + carbon dioxide. Some other bacteria have red chlorophylls and can use light and carbon dioxide to make the sugars they need. Like all living things bacteria require mineral salts such as magnesium, calcium, iron, and others. Some bacteria are able to obtain the energy they need by oxidizing iron or sulfur. Some bacteria need sugars, vitamins, aminoacids, and other growth factors already digested and ready to use. Other bacteria can digest proteins down to amino acids and digest complex carbohydrates such as starches and table sugar down to simple sugars. Some bacteria can make their own amino acids and vitamins from carbohydrates. The metabolic abilities of bacteria are among the traits we use to group them into genera.

Bacteria species differ greatly in the conditions they need for growth. Some grow best in cool places such as soil or bodies of water, but others are able to grow in hot springs, hot water heaters, or undersea volcanoes. The bacteria which cause disease in mammals and birds, usually grow best at body temperatures. Many bacteria which cause diseases in hydra, snakes, turtles, and other cold-blooded animals, are not able to cause disease in birds or mammals because the high body temperatures kill these bacteria or limit their growth. This web site offers several pages of experiments to study the effect of pH, temperature, osmotic pressure (salt and sugar concentrations), oxygen concentration, and other environmental factors upon the growth of bacteria.

As you can imagine from this discussion, the numerous species of bacteria live in an astonishing variety of places and live on every food you can imagine. Some can eat gasoline and other hydrocarbons. In fact, machinists face disagreeable odors on Monday morning from the accumulated waste products of microbes living in the cutting oils used to lubricate drills and cutting tools. We often say termites, cows, beavers, beetle larva, and other organisms can eat wood or sawdust. Actually, it is usually bacteria in their guts that eat the wood and the animal lives on the dead bacteria and substances they produce. In many instances, the bacteria actually live inside protozoa which live in the gut of the wood eating animal.

If some new substance comes along, a bacterium may have a little ability to eat it and that gives it a slight competitive edge because he can live when the others have run out of food. It may slightly outgrow his neighbors. Over time minor changes in genetic material occur which are called mutations. If these minor changes help an organism to grow better than its competitors, that organism may outgrow his neighbors. That is one reason the balance between living things on earth slowly changes. Mutations are constantly occurring. Nearly all mutations are harmful, meaning the change is not quite as good as the original, but some mutations give an organism an advantage over his neighbors. Genetics is the science of studying genetic material. Bacteria are good subjects for studying the basic laws of inheritance because they grow rapidly and and have traits which are easily studied.

Bacteria are important in food preparation and preservation. Firstly, many species would love to eat our food and we must find a way to keep them out or slow their growth. For example, foods retain their value longer when frozen because few bacteria can grow in a freezer, and the low temperature also slows biochemical reactions. Secondly, certain bacteria have specialized metabolisms which are ideal for food fermentations. For example, man has found many species which can digest sugars but not proteins. These species are very useful in food preparation if they produce acids which slow the growth of other organisms which would eat the proteins. Bacteria and yeasts which produce alcohols preserve foods because the alcohol kills other organisms or slows their growth. Primitive peoples around the world have developed an interesting variety of fermented foods: cheeses, drinks, breads, fish pastes, nut pastes, and others. These primitive foods sometimes cause disease because they contain pathogens such as TB, or toxic substances. Food microbiologists interested in producing safe versions of the primitive foods begin by isolating the microbes which cause the desired reactions and flavors. Then they add pure cultures of these organisms to clean raw food to produce safe, wholesome cheeses, breads, and other foods.

From the above, it is clear that the detailed study of bacteria may involve studying chemistry, botany, zoology, genetics, biochemistry, and other sciences. Even the use of mathematics and computers can enhance your bacteria science project. This web site will provide information and links to all those areas. Just begin something and let your interests lead you where they may.

Microbial safety precautions should be considered from the very beginning of your microbiology project. Most bacteria are harmless and many of those living on our skin help protect us and many species living in our gut are beneficial. For example, bacteria in the human gut produce vitamin K which is essential for normal clotting of blood.

Be sure to read our safety pages before you begin any experiments. Discuss your project with your parents and teachers and work out a safety plan before you begin. Helping your parents prepare and cook ordinary foods will give you valuable safety lessons because cuts, burns, and abrasions are the most common injuries in microbial projects.

Click here to go to a list of the pages in this site devoted to bacteria.

Click here to go to a list of WWW sites having information about bacteria. All these links worked when this page was written; if you find problems, or know of other useful web sites, please send me an e-mail.

Written by Harold Eddleman, Ph. D., President, Indiana Biolab, 14045 Huff St., Palmyra IN 47164
©All material is protected by Section 107 of the 1976 copyright law.
Copyright 1997 by Harold Eddleman. Copyright law permits you to use this material in teaching and self-study. If you intend to use this material in teaching, please acknowledge the author and the source of the information. When you give your students the URL, you enable them to get updates.

B120 - Isolation of pure cultures of bacteria from human foods by students at home.
B121 - Principles of isolating and using pure cultures of bacteria in production of fermented foods.
B122 - Isolation of Brevibacterium linens from Limburger Cheese at home by a beginner.
B203 - Isolation and characteristics of Vibrio (Photobacterium) phosphoreum, glows brightly.