Symbiotic Relationships Between Microbe and Host

The term symbiosis means "to live together." This long-term association between individuals of different species is often misinterpreted as always being a positive type of relationship, where both organisms benefit. This is not true. There are actually three types of symbiosis, with the interactions between organisms ranging from win-win to win-lose.

Symbiosis, Normal Flora and Pathogens

There are billions of microbes that have symbiotic relationships with people; some live on the human body full time, others are occasional interlopers. Normal flora are the microbes that live in and on the human body and are beneficial or neural in their effect on the host. Therefore, normal flora are either mutualistic or commensalistic. Microbes that cause human disease, have a parasitic relationship with their host, and are referred to as pathogens. The following covers the main types of symbiotic relationship that a microbe may have with its host.

Mutualistic Symbiosis

This is the win-win type of symbiosis, in which both organisms benefit from cooperating. An example of a mutualistic relationship between a microbe and host is the normal flora that live in the human intestine. The bacteria get a place to live complete with a built in food source and in return they benefit the host by aiding digestion and producing vitamin K.

Commensalism

Commensalistic relationships occur when one organism benefits while the other is not much effected for better or worse. Staphylococcus epidermidis is a Gram+ bacteria that is found on the surface of the skin. When living on the skin, these halophilic (salt-loving) bacteria have a surface to grow on and are able to consume waste products that the skin generates. Staph epi are normal flora of the skin, that do not harm or directly help the host, although they do take up space on the skin, crowding out potentially pathogenic (harmful) bacteria.

Parasitic Symbiosis

Parasitism takes place when one organism benefits at the cost of the other. A parasite that causes disease in its host is called a pathogen. There are many microbes that are parasitic to humans, including fungi, protozoans, bacteria and viruses. One example of a parasitic bacteria would be the acidophile Helicobacter pylori, which can cause infection in the lining of the stomach that often results in peptic ulcers.

The human body is essentially an ecosystem for billions of symbiotic microbes; with the nature of the relationship running the gamut from beneficial to pathogenic.

Sources
Bauman R. (2007) Microbiology with Diseases by Taxonomy, Second Edition. Pearson Benjamin Cummings.Todar's Online Textbook of Bacteriology

The Virtual Microbiology Classroom