Microbiologists study all critters small and tiny. These critters include viruses, bacteria, fungi, and parasites. There is another group of agents called prions (one such prion causes Mad Cow Disease) however we will get into that subject later. This article is designed to give you a virtual visual tour of the microorganisms (those little critters) from smallest to largest.
Viruses are the smallest things we study. They are very wimpy. They all require another living cell to reproduce. I would call them the ultimate parasite. They take all they need from the cells they live in and when done they usually destroy the host cell. Don't they sort of remind you of a teenager (Just kidding)? An example of how small they are can be found at the Virus Ultrastructure site. This site also contains a number of viral images. Viruses come in a number of shapes and sizes. To see a virus you need a very special microscope called an electron microscope.
Bacteria are the next size up. They are much larger than viruses (10 times larger). Go look at Cells alive. It has a great webpage comparing the size of viruses to bacteria. Bacteria like viruses are all around. However, most of these organisms can reproduce without help. Only of few bacterial groups need help to reproduce (examples Rickettsia, Erhlichia).
There are many many different bacteria and they come in a variety of shapes and sizes. They can be round (cocci), stick-like (rods), and spiral shaped. Usually you need a light microscope to see bacteria. However, there is one bacteria that is so large you can see it without using a microscope(Epulopiscium fishelsoni)! The Microbe Zoo is a great place to see a variety of bacteria and discover where they can be found. You can see a rod-shaped bacterium called Escherichia coli. You can also see a spiral shaped organism called Leptospira interrogans. I study bacteria and I am most interested in the bacterium called Staphylococcus aureus.
The next group of organisms we study are the fungi. They include mushrooms, molds, and yeasts. However, microbiologists usually only study the molds and yeasts. We leave the larger fungi to the biologists. Yeasts are still single celled organisms like the bacteria. The yeast cells are usually about 5-10 times bigger than bacteria but you still require a microscope to see them. That fuzzy stuff (mold) you see on moldy bread is a multicellular fungus and can form quite large colonies (1-5 inches across). In fact an antibiotic called penicillin is obtained from one of those nasty old bread molds. To see any details however still requires a microscope. Examples of the molds and yeast can be seen at Mycology Online.
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