What is Symbiosis?

Symbiosis is an intimate association between two different organisms. In fact, most animals and plants live symbiotically with microorganisms. The larger organism is called the host and smaller organism or organisms the symbionts. Examples include bacterial colonization of the skin and digestive tract of animals and the roots of plants. For the microorganism, the benefits of the association can be a stable protective environment provided by the host. The bacteria may also obtain nutrients from the host. On the other hand, the symbionts can "protect" the host by making it more difficult for colonization by pathogenic bacteria. Some symbionts supply the host with nutrients that the host cannot synthesize themself nor obtain from their food.

The original definition of symbiosis by deBary (1879) did not include a judgment on whether the partners benefit or harm each other. Currently, most people use the term symbiosis to describe interactions between the symbiont and the host from which both partners benefit; this is also called a mutualism. If there is negative effect on one of the partners, it is called a parasitic symbiosis and if there is no beneficial or negative effect it is a commensal symbiosis. These clear-cut definitions are not always easy to apply in nature. Take the bacterium Pseudomonas aeruginosa for example. This bacterium can be found on the skin of humans and not cause disease, perhaps we would call it a commensial, but if the person has a severe burn P. aeruginosa can cause an infection and becomes a pathogen (a medicinal term for parasitism). This type of organism is called an opportunistic pathogen.

Whether an association is a mutualism, commensialism or parasitism depends on the relative "strengths" of the partners and the balance of power can change over time. For examples, go back to our bacteria animal symbiosis home page.


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