Symbiosis is a relationship between two or more organisms that live closely together. There are several types or classes of symbiosis:
- One organism benefits and the other is neither harmed nor helped.
- Both organisms benefit. An obligate mutualist cannot survive without its partner; a facultative mutualist can survive on its own.
- One organism (the parasite) benefits and the other (the host) is harmed.
To be successful, a symbiotic relationship requires a great deal of balance. Even parasitism, where one partner is harmed, is balanced so that the host lives long enough to allow the parasite to spread and reproduce.
These delicate relationships are the product of long years of co-evolution. Bacteria were the first living things on the planet, and all of Earth's other creatures have been living and evolving with them for hundreds of millions of years. Today, microbes are essential for many organisms' basic functions, including nourishment, reproduction, and protection.