Symbiosis is generally defined as a condition where two dissimilar organisms live together in an intimate associate that sees both organisms benefit. Microbial symbiosis tends to be bit broader in definition, being defined as the co-existence of two microorganisms .
Microbial symbiosis can be evident as several different patterns of co-existence. One pattern is known as mutualism. In this relationship, both organisms benefit. Another type of relationship is called commensalism. Here the relationship is beneficial to one of the organisms and does no harm to the other.
Another relationship known as parasitism produces a benefit to one organism at the expense of the other organism. Parasitism is not considered to be a symbiosis between a microorganism and the host.
Microbial symbiosis has been a survival feature of bacteria since their origin. The best example of this is the presence of the energy factories known as mitochondria in eukaryotic cells. Mitochondria arose because of the symbiosis between an ancient bacterium and a eukaryote. Over evolutionary time the symbiosis became permanent, and the bacterium became part of the host. However, even to the present day the differences in constitution and arrangement of the genetic material of mitochondria and the host cell's nucleus attests to the symbiotic origin of mitochondria.
There are several well-known examples of bacterial mutualism. The first example is the presence of huge numbers of bacteria in the intestinal tract of warm-blooded animals such as humans. Fully 10 percent of the dry weight of a human consists of bacteria. The bacteria act to break down foodstuffs, and so directly participate in the digestive process. As well, some of the intestinal bacteria produce products that are crucial to the health of the host. For example. In humans, some of the gut bacteria manufacture vitamin K, vitamin B 12, biotin, and riboflavin. These vitamins are important to the host but are not made by the host. The bacteria benefit by inhabiting an extremely hospitable environment. The natural activities and numbers of the bacteria also serve to protect the host from colonization by disease-causing microorganisms. The importance of this type of symbiosis is exemplified by the adverse health effects to the host that can occur when the symbiotic balance is disturbed by antibiotic therapy.
A second example of symbiotic mutualism is the colonization of the nodules of leguminous plants by bacteria of the genus Rhizobium. The bacteria convert free nitrogen gas into a form of nitrogen called nitrate. This form of nitrogen can be readily utilized by the plant, which cannot otherwise use the gaseous form of nitrogen. The plant benefits by acquiring a readily available nitrogen source, and, as for the intestinal bacteria, Rhizobium benefits by virtue of the hospitable environment for growth.
The skin is colonized by a number of different types of bacteria, such as those from the genera Staphylococcus and Streptococcus. The bacteria are exposed to a read supply of nutrients, and their colonization of the skin helps protect that surface from colonization by less desirable microorganisms.
Microbial symbiosis can be exquisite. An example is the Gram-negative bacterium Xenorhabdus nematophilus. This bacterium lives in a nematode called Steinernema carpocapsae. Both organisms require the other for their survival. Thus the symbiosis is obligatory. The bacterium in fact supplies toxins that are used to kill insect that the nematode infects.
The scope of microbial symbiosis in nature is vast. In the 1970s the existence of thermal vents on the ocean floor was discovered. It has since been shown that the basis of the lush ecosystem surrounding these sources of heat is bacteria, and that a significant proportion of these bacteria live in symbiosis with the tubular worm-like creatures that thrives in this environment. In fact, the bacteria are absolutely required for the utilization of nutrients by the tube worms.
Numerous other examples of microbial symbiosis exist in nature. Animals, plants as exotic as coral, insects, fish, and birds all harbor microorganisms that assist them in their survival. Indeed, the ancient roots of microbial symbiosis may be indicative of a more cooperative evolution of life on Earth than prior studies indicated.
See also Bacterial kingdoms; Microbial taxonomy