Domain Bacteria is composed of organisms that are much more common than Archaea and live almost anywhere. There are more bacteria in a person's mouth than there are people in the world. Many are decomposers, some are photosynthesizers, and a few cause disease. Most bacteria cause disease by producing exotoxins that harm human cells, while others cause illness as a result of glycoproteins found on the outside of their capsules. There are many shapes that bacteria can come in, but three of the main ones are cocci, bacilli, and spirochete. Cocci are spherical prokaryotic cells that are usually found in clusters or colonies. Some cocci form chains; bacteria of this type are called streptococci. Bacilli are rod shaped cells that are usually found individually, although a few form pairs, diplobacilli, or chains, streptobacilli. Spirochetes are the last group. The bacteria in this group are helically shaped and usually are found alone. They can reach .5 mm long but are extremely thin.
To see the ecological and practical roles of bacteria in our society today, click here.
To see how bacteria generated eukaryotic cells through endosymbiosis, click here.
There are five phyla of bacteria: Proteobacteria, Cyanobacteria, Gram-Positive Eubacteria, Spirochetes, and Chlamydiae.