Quite a few mutualistic symbioses involving prokaryotes are known, but unfortunately they don't get nearly the attention they deserve. Without their existence, however, life as we know it would not exist. Let's consider a few examples.
Nitrogen is essential to all life. It is required for the synthesis of nucleic acids, proteins, and a host of other important biomolecules. However, without the action of certain prokaryotes, little nitrogen would be available to the biosphere. There are several species of bacteria that can convert atmospheric nitrogen (N2 ) into a form (e.g., ammonium) that can be used by other life. One of the most important groups are those bacteria that form a symbiosis with certain plants. For example, Rhizobium bacteria colonize on the roots of pea plants, where they fix nitrogen. The plant provides carbon to the bacteria, and the bacteria provide nitrogen to the plant, a classic and extremely important example of a mutualistic symbiosis. We will discuss this further in the section on the Nitrogen Cycle.
Humans serve as a host for a number of mutualistic symbioses with prokaryotes. Researchers have studied these to varying degrees. For example, one (or more) of the bacteria species that colonize our lower gastrointestinal tract has the ability to synthesize vitamin K (which humans cannot produce). Vitamin K is taken up by your intestines and used in blood-clotting (and possibly other reactions). In this symbiosis, humans get an essential vitamin, and the bacteria get a source of carbon from the food we do not completely digest.
In a parasitic symbiosis, one member of the symbiosis benefits at the expense of another. Parasitic bacteria that cause disease in their hosts are called pathogens. These pathogenic bacteria cause disease by either invading healthy host tissue or by producing toxins that poison the host.
Commensalism is defined as a relationship between two organisms in which one benefits and the other is neither harmed nor helped. Many of the bacteria living on the surface of your skin are an example of commensalism. But remember, these relationships can quickly change - if you cut or scratch yourself, bacteria can move from the outer surface of your skin to inside your body and cause an infection.