Organisms are constantly interacting with one another in various ways. A symbiosis is an ecological relationship between two or more organisms that live in direct contact with one another. Several types of symbiotic relationships are classified based upon the benefit or harm to the organisms involved. The boundaries of symbiotic types (mutualism, synergism, commensalism, parasitism, and antagonism) are fluid. Interactions among organisms tend to exist on a continuum of benefits and harms. With one set of circumstances, a mutualistic relationship may form between two organisms, but under a different set of circumstances, one of the organisms may opportunistically take advantage, as in a parasitic relationship.
For example, mycorrhizal fungi and plants are mutualists. Fungi colonize the root system of plants, and in doing so, their hyphae (the small filaments that make up a fungal body) increase the surface area available for absorption. This results in increased water and nutrient (such as nitrogen and phosphorus) absorption to the plants. The fungi also protect the plants against certain pathogens. In turn, the plants provide the fungus with the carbohydrates produced from photosynthesis.
Some plants, such as those in genus Cytinus, form parasitic associations with other plants and their mycorrhizal fungi. These parasitic plants through evolution lost the ability to produce their own food from photosynthesis. The parasitizing plant will attach to the host plant's roots, which contain the mycorrhizal fungi, and absorb the water, carbohydrates, phosphorus, nitrogen, and other nutrients from the host plant and fungi.
Mutualism, Commensalism, and Synergism
Mutualism is a relationship in which both organisms benefit from the relationship. Bacteria in the human intestines are in a mutualistic relationship with their human host: the microbes have a place to live and food to eat, and the human is assisted in digestion and receives vitamins in return. The human gut is populated with Lactobacillus (the bacteria in probiotic foods, such as yogurt), E. coli, and several Bacteroides species, among others. Collectively, these bacteria produce vitamin B12 and vitamin K for their human hosts. In addition, the presence of these bacteria helps to control the growth of harmful bacteria by competing for the same food source. The higher the number of probiotic bacteria in the intestines, the less space and nutrient availability for infection-causing bacteria, such as Clostridium difficile, to grow. Many bacteria also help to break down the cellulose of plant matter with the help of CAZymes, or carbohydrate-active enzymes.
Commensalism is a relationship in which one species benefits but the other neither benefits nor is harmed. Staphylococcus species live on human skin, where they are provided with nutrients. Therefore, the Staphylococcus species is benefitted, but the human host does not benefit and is not usually harmed from this relationship. However, like some commensal organisms, Staphylococcus can become pathogenic (if host immunity is low or there is a break in the skin barrier) and cause disease.Synergism is a relationship among organisms that allows for the creation of an effect that neither could create on its own. The relationship is not obligatory, and both organisms can survive independently. One example of synergism involves the bacterium Pseudomonas. Pseudomonas produces a waste product, orcinol, which inhibits its growth. However, if other bacteria are present, they benefit by feeding on the orcinol while the Pseudomonas benefits because its growth is uninhibited.
Parasitism is a symbiotic relationship where one organism benefits to the detriment of the other organism. Microsporidia is a parasite that can be found living in the cells of just about every group of organisms. These fungal parasites have been known to cause skin and muscle infections, diarrhea, and urinary tract infections. Plasmodium is unicellular parasite that causes malaria. It is spread through the saliva of female mosquitoes and can cause severe fever, aches, and pains. Another type of parasitism involves the infection of a host organism that ultimately results in the death of the host. In this case, the parasite is specifically called a parasitoid because the host dies. Ophiocordyceps unilateralis is a fungus found in tropical forests. Its life cycle involves parasitizing the bodies of ants by growing and reproducing inside the insects' bodies. In this way, the fungi have access to food and a place to reproduce inside the insect, but the insect dies as a result of this interaction. Human beings can be similarly parasitized. A tapeworm, or Taenia solium, a type of flatworm, can be ingested by a human host through consumption of infected meat. The tapeworm travels to the human intestine and attaches itself there, depriving its human host of vital nutrients.
A related concept to parasitism is antagonism, which is a competitive relationship among organisms where one inhibits or suppresses the growth of others. Many strains of bacteria are antagonistic to others, including E. coli, a resident bacterium of the large intestine. E. coli produce and release bacteriocins, which are proteins that specifically target certain harmful bacteria, such as Shigella or Salmonella species. Because antagonistic relationships do not depend on direct interaction between organisms, some scientists do not consider the relationships truly symbiotic.
Types of Symbiotic Relationships
|Type of Symbiosis||Example||Description|
|Mutualism||Gut microflora and animals||Some of the bacteria and yeast that inhabit the digestive tracts of animals are provided with living space and food. In turn, the animal host is aided in digestion.|
|Plants and mycorrhizal fungi||Some types of fungi called mycorrhizae colonize the roots of plants. The plant provides the fungi with sugar "food," and in return the fungi help the plant acquire nutrients and water.|
|Synergism||Humans and Bacteroides, Eubacterium, Propionibacterium, and Fusobacterium||These bacteria feed off food provided in the colon of their human host. In return, they produce vitamin K, folate, vitamin B12, and biotin. In addition, they digest otherwise indigestible plant matter for their human hosts.|
|Escherichia coli and Streptococcus faecalis||Streptococcus faecalis produces ornithine as a byproduct of metabolism, which Escherichia coli cannot produce on its own. However, when in the presence of ornithine, E. coli takes up this molecule to synthesize proteins. In turn, E. coli produces putrescine, which S. faecalis cannot synthesize, yet it serves as a nutrient for S. faecalis. Together, both bacteria produce a molecule that the other can use in its own metabolism.|
|Commensalism||Staphylococcus aureus and humans||Over 1,000 species of bacteria live on the skin of healthy human beings, one of which is Staphylococcus aureus. The Staphylococcus benefits by feeding on dead skin cells and oils, and the human host is neither harmed nor benefitted.|
|Demodex mite and humans||Demodex folliculorum is a mite that lives within human hair follicles. There, it feeds on the cells that line the follicle in addition to sebum. The human host is neither harmed nor benefitted.|
|Parasitism||Ophiocordyceps fungi and insects||Fungi in the genus Ophiocordyceps grow inside insects, killing them before growing their fruiting bodies out of the insects' exoskeleton.|
|Yersinia pestis and humans||Any bacterium that can infect a host, multiply within, and harm that host can be considered a parasite. Yersinia pestis is a gram-negative coccobacillus responsible for the bubonic plague. It spreads to human hosts via a flea vector, and its virulence is linked to its antiphagocytic capabilities. It proliferates inside lymph nodes and destroys immune cells such as macrophages.|
|Antagonism||Antibiotic-producing bacteria||Many bacteria produce antibiotics. They secrete these antibiotics into their environment, where they inhibit the growth of neighboring bacteria, effectively increasing the area they can colonize while reducing that of other species.|
|Protozoa and prokaryotes||Protozoa, particularly bacterivorous nanoflagellates, play a significant role in decreasing bacterial populations in aquatic ecosystems through predation of bacterial species. This process is called grazing.|