Symbiosis: Definition, Types and Ways Organisms Live Symbiotically

It can be defined as the coexistence or the intimate association of two or more different organisms.

In this close association between two different organisms, at least one organism benefits.

In fact, most animals and plants live symbiotically with microorganisms .

Examples include bacterial colonization of the skin and digestive tract of animals and the roots of plants.

For the microorganism, the benefits of association may be a stable protective environment provided by the host. Bacteria can also obtain nutrients from the host.

On the other hand, symbionts can “protect” the host by making it difficult for pathogenic bacteria to colonize.

Some symbionts provide the host with nutrients that the host cannot synthesize or obtain from their food.

Different types of symbiosis

Organisms interact with each other in a variety of ways.

These interactions can be cooperative, antagonistic, defensive, reciprocal, harmful, community, opportunistic, beneficial, or neutral.

Symbiosis represents the relationships that different species of organisms have with each other.

Communities consist of many different populations of species that occupy the same niche and that can interact in one way or another.

While symbiotic relationships can involve more than two organisms, usually only two organisms are considered to simplify the explanation.

In all these relationships, at least one party benefits, it is what happens to the other party that makes things more interesting.

The original definition of symbiosis did not include a judgment on whether the partners benefit or harm each other.

Currently, most people use the term symbiosis to describe the interactions between the symbiote and the host from which both partners benefit, this is also called mutualism.

If there is a negative effect in one of the couples, it is called parasitic symbiosis and if there is no beneficial or negative effect it is commensalism.

In the case of the bacterium Pseudomonas aeruginosa for example.

This bacteria can be found on human skin and not cause disease, perhaps it could be called commensalism, but if the person has a severe burn, Pseudomonas aeruginosa can cause an infection and become a pathogen (a medicinal term for parasitism. ).

Whether an association is mutualism, commensalism, or parasitism depends on the relative “strengths” of the partners, and the balance of power can change over time.

According to current studies, a molecule produced naturally by symbiotic gut bacteria could offer a new type of treatment for inflammatory bowel disease.

The 100 trillion bacteria that occupy the human gut have evolved alongside the human digestive and immune systems over millions of years.


In a mutual relationship, all parties benefit.

A case of mutualism is represented by a type of intestinal flora that lives in the stomach of human beings.

They receive food, but they also help to digest food properly.

It is surprising when you consider that the human body contains trillions of cells that are not human but microbial.

Like human cells, these microorganisms have genes that instruct their cellular activity.

This massive microbial community that inhabits the human body and its collection of genes is called the human microbiome.

While the human microbiome is primarily made up of bacteria, it also includes numerous diverse single-celled organisms called archaea, as well as fungi, and even viruses.

Wherever the human body is exposed to the outside world, microbial communities reside.

The skin, nasal passages, mouth, intestines, colon, and urogenital tract are several areas of the body where these microorganisms normally settle.

From a biological point of view, a diverse ecosystem is generally more resilient.

And, people with large and diverse bacterial populations in their digestive tracts tend to be less prone to obesity, immune problems, and other health problems than those with decreased microbial diversity.

Beneficial bacteria help extract nutrients from the food you eat as you eat complex carbohydrates that your body cannot digest.

The final products, resulting from digestion, help feed the cells that line the intestinal wall and help the body absorb minerals such as; calcium, magnesium and iron.

Microorganisms in the gut also help make some vitamins, including vitamin K, vitamin B12, and biotin.

Bacterial species remain remarkably the same over time, environmental conditions such as stress, drugs, gastrointestinal surgery, infections, and toxic agents can alter this microbial balance.

The most common cause of dysbiosis, which is the imbalance of bacteria in the microbiome, is antibiotics.

A healthy diet includes complex carbohydrates (vegetables and fruits) in addition to lean meat and the main unsaturated fats.

But, if you want to ensure a robust microbiome, make sure you eat plenty of foods that contain prebiotics.

These indigestible fibers nourish beneficial bacteria and can be found in a number of vegetables, fruits, grains, and roots.

Good sources of prebiotics include artichokes, asparagus, bananas, berries, carrots, garlic, jicama, leeks, legumes, onions, radishes, and tomatoes.


In parasitism, an organism benefits while one is harmed. In a parasitic relationship, one party pays a price while one party gets all the benefits.

Parasites can reside in a host organism and as such can be classified into two categories: ectoparasites and endoparasites.

Ectoparasites, as their name implies, are found outside the host, on the skin, such as fleas, lice, ticks. In contrast, endoparasites are found within the host, such as tapeworms.

A characteristic of this type of association is that one organism is usually smaller than the other. The smallest in this case is the parasite, and the largest is the host.

The smallest benefits, and the largest suffers. Parasites, whether ectoparasites or endoparasites, can have intermediate hosts.

These intermediates do not necessarily face any negative consequences. Vectors are often used by parasites to get from point A to point B. As a result, these relationships can be somewhat complex.

A typical case to demonstrate is the success of the malaria virus, due to a successful complex symbiotic relationship. Malaria is transmitted by Anopheles mosquitoes .

These mosquitoes do not contract the infection, but the host to which they are transferred (a human) catches it and is adversely affected.

The mosquito, in this case only females, since the males do not feed on blood, normally sucks the blood of a human.

The virus-carrying mosquito inadvertently deposits the parasite into the bloodstream. And there the virus develops infecting the host.

Ectoparasites in humans

There are a variety of ectoparasites in humans, affecting various parts of the body and causing disease:

  • The bed bug, or cimicidae, can attack anywhere on the body and causes skin irritation.
  • Head lice remain on the scalp, causing pediculosis capitis.
  • Body louse or Pediculus humanus, can lodge in any part of the body and causes a pediculosis.
  • Crab louse or Pthirus pubis, is located in the pubic area giving rise to Pediculosis pubis.
  • Demodex, lodges in eyebrows and eyelashes and causes demodicosis.
  • Scabies, or Sarcoptes scabiei, can attack any part of the skin and produce an itchy rash.
  • Human flea or Pulex irritans can also lodge anywhere on the skin and cause itching and inflammation

A common example of parasitism with humans is malaria infections caused by mosquitoes. Mosquitoes depend on humans for food. They drink human blood to help care for their eggs.

When the mosquito is sucking the human’s blood through the long suction mouthpiece, the mosquito’s saliva enters the human’s bloodstream directly.

This saliva infects the human body with microorganisms and can cause many different diseases, such as malaria, yellow fever, filariasis, elephantiasis, and heartworm.

This human-mosquito relationship would be considered parasitism because the human (host) is adversely affected and the mosquito (parasite) benefits from the relationship.

The mosquito gets food but, on the other hand, the human gets sick and can even die.

Endoparasites in humans

There are a wide variety of ectoparasites that live in humans, and that affect various parts of the body and cause diseases such as:

  • Acanthamoeba: encefalitis amebiana granulomatosa.
  • Balamuthia mandrillaris: encefalitis amebiana granulomatosa.
  • Babesia: babesiosis.
  • Balantidium coli: balantidiasis.
  • Blastocystis: blastocistosis
  • Cryptosporidium: criptosporidiosis.
  • Dientamoeba fragilis: dientamoebiasis.
  • Entamoeba histolytica: amebiasis.
  • Giardia lamblia: giardiasis.
  • Isospora belli: isosporiasis.
  • Leishmania: leishmaniasis.
  • Naegleria fowleri: meningoencephalitis amebiana primaria.
  • Plasmodium falciparum: malaria.
  • Rhinosporidium seeberi: rhinosporidiosis.
  • Sarcocystis: sarcosporidiosis.
  • Toxoplasma gondii: toxoplasmosis.
  • Trichomonas vaginalis: trichomoniasis.
  • Trypanosoma brucei: African trypanosomiasis or sleeping sickness.
  • Trypanosoma cruzi: Chagas disease.
  • Diphyllobothrium: diphyllobothriasis.
  • Echinococcus granulosus: hydatid disease.
  • European hedgehog spirometer: asparganosis.
  • Taenia saginata: taeniasis.
  • Tape throne cisticercosis.
  • Fasciola hepatica: fascioliasis.
  • Metagonimus yokagawai: metagonimiasis.
  • Schistosoma: schistosomiasis.
  • Swimmer’s itch or cercarial dermatitis : schistosomatidae.
  • Paragonimus westermani: paragonimiasis.
  • Ancylostoma duodenale (ankylostomiasis): ankylostomiasis.
  • Enterobius (lombriz intestinal): enterobiasis.
  • Thelazia callipaeda: thelaziasis.
  • Trichinella spiralis: triquinosis.
  • Trichuris trichiura: trichuriasis.


In commensalism, one organism benefits while the other organism is unaffected.

In a commensal relationship, however, one party benefits while the other party does not gain but does not pay any price.

The term “diner” is derived from a Latin word Mensa, which means “table.” Commensal organisms share their food from a common table.

In the case of human diners, the human host is the table.

While in utero, humans live in a sterile environment, protected by the placenta and the amniotic sac.

After birth, humans are exposed to a variety of new organisms.

If these organisms are in a suitable ecological niche, either in the internal or external parts of the human body, they will multiply and form complex communities, or colonies, with their host.

They will form lasting, stable and interdependent relationships with other organisms in the same place and with the human being that harbors them.

Since the benefit to one species or another can be subtle and difficult to identify, it is often difficult to distinguish between true commensalism and mutualism.

Humans certainly derive considerable benefit from many resident organisms.

Scientists estimate that the human body has about 7.5 x10 13 cells.

Many of these cells are not of human origin, but rather represent commensal and mutual microscopic organisms.

From the moment of birth and throughout our lives, humans share their bodies with a surprising variety of microscopic organisms.

Dust mites remove dead skin, amoebae live on your teeth, and remove food particles.

For example, microscopic mites that live in the eyebrows, eating the dead skin cells that shed, receive food, but do not affect the host.

A wide variety of microorganisms interact with humans, taking advantage of various microenvironments.

Certain parts of the body, such as solid organs, blood, cerebrospinal fluid, and urine, are normally sterile.

However, established microbial populations can be found on the skin and in the lower respiratory tract, mouth, and lower intestine.

Throughout life, these resident organisms vary in type and number, and people can have significant differences in their resident populations.

If diners gain access to inappropriate body sites for any reason, they can lead to infections.


Symbiosis plays a central role in the creation of diverse, interesting and complex life forms.

Species live together, share resources, and occasionally help each other, this is symbiosis.

Helping each other is what symbiotic relationships are all about, specifically relationships of mutualism.

Sometimes one species helps itself at the expense of the other.

And this is called parasitism, however, sometimes we find a species that helps itself, and the other organism involved is not affected at all.

In fact, in some cases, the body does not even notice it, this is commensalism.

Clearly, the interaction of species, fundamentally symbiotic relationships, is crucial for prokaryotic and eukaryotic diversity.