BACTERIA
Unlike viruses, bacteria display all of the characteristics associated with living organisms. Bacteria play an important role in the recycling of nutrients, photosynthesis, nitrogen fixation, and bioremediation. Most bacteria are beneficial to humans; however, some bacteria are pathogenic (i.e., able to cause illness).

Bacteria possess a number of structures that allow them to attach to target surfaces or tissues. Many bacteria are surrounded by a capsule, which is composed of polysaccharides and glycoproteins. The capsule is important in bacteria that cause disease because it prevents phagocytosis by the host's white blood cells. The sticky nature of the capsule also allows the bacterium to bind to target surfaces. For example, the slimy feel of your teeth in the morning is caused by the capsules of bacteria that have attached to your teeth. Another factor that makes the capsule important is that other microorganisms can stick to the capsule, resulting in the formation of a biofilm (a community of microorganisms).