Does parasitism, one of Bacteria's lifestyle?

Does parasitism, one of Bacteria's lifestyle?

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The parasite is an organism that lives in or on a host. It depends on its host for survival. Bacteria lives in decaying organic matter, within human organism (colon, oral cavity). It can be a saprophyte or commensal.

Does the parasitism, one of bacteria's lifestyle ?

If you are asking whether some bacteria are parasites, then absolutely yes. I think this is absolutely clear and evident to any and all biologists and would require no discussion. I could expand with some examples, but they would not be very representative of the incredible diversity of parasitic bacteria.


Many bacteria parasitize and live selfishly on hosts. Among them are obligate and facultative parasites. Many become parasitic if there are changes on their host (e.g. think pH in your oral cavity). These are endless. And to expand on this ecological discussion, I think another related and interesting relationship in nature exists which is not technically parasitic but falls into a similar category…


Yet many other bacteria are symbiotic commensals, i.e. do not benefit nor harm the host. As commensals, they have various commensal relationships with the host:

  1. chemical commensals

(exploit the available chemistry of the host),

  1. inquilinism

where the bacteria will live in the host's prepared environment (think ant hills or termite mounds or bird nests; or sewers, air vents, the list is endless especially for human environments)

  1. metabiosis

(when the aprasite depends on the environment previously prepared by another organism; think how bacteria may parasitize fungi on the ground, which digest dead plant matter; the fungi deploy antibiotics to prevent this kind of parasitism, and we've used these defences for our own medicine… penicillin is from fungi!), and

  1. phoresy.

What to know about parasites

A parasite is an organism that lives in another organism, called the host, and often harms it. It depends on its host for survival.

Without a host, a parasite cannot live, grow and multiply. For this reason, it rarely kills the host, but it can spread diseases, and some of these can be fatal.

Parasites, unlike predators, are usually much smaller than their host and they reproduce at a faster rate.

Share on Pinterest Parasites range from microscopic in size to over 30 meters in length.

A parasite is an organism that lives within or on a host. The host is another organism.

The parasite uses the host’s resources to fuel its life cycle. It uses the host’s resources to maintain itself.

Parasites vary widely. Around 70 percent are not visible to the human eye, such as the malarial parasite, but some worm parasites can reach over 30 meters in length.

Parasites are not a disease, but they can spread diseases. Different parasites have different effects.


These live inside the host. They include heartworm, tapeworm, and flatworms. An intercellular parasite lives in the spaces within the host’s body, within the host’s cells. They include bacteria and viruses.

Endoparasites rely on a third organism, known as the vector, or carrier. The vector transmits the endoparasite to the host. The mosquito is a vector for many parasites, including the protozoan known as Plasmodium, which causes malaria.


These feed on other parasites in a relationship known as hyperparasitism. A flea lives on a dog, but the flea may have a protozoan in its digestive tract. The protozoan is the hyperparasite.

There are three main types of parasites.

Protozoa: Examples include the single-celled organism known as Plasmodium. A protozoa can only multiply, or divide, within the host.

Helminths: These are worm parasites. Schistosomiasis is caused by a helminth. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke.

Ectoparasites: These live on, rather than in their hosts. They include lice and fleas.

There are many types of parasite, and symptoms can vary widely. Sometimes these may resemble the symptoms of other conditions, such as a hormone deficiency, pneumonia, or food poisoning.

Symptoms that might occur include:

  • skin bumps or rashes
  • weight loss, increased appetite, or both
  • abdominal pain, diarrhea, and vomiting
  • sleeping problems
  • aches and pains
  • allergies
  • weakness and general feeling unwell

However, parasites can pass on a wide variety of conditions, so symptoms are hard to predict.

Often there are no symptoms, or symptoms appear long after infection, but the parasite can still be transmitted to another person, who may develop symptoms.

Many types of parasites can affect humans. Here are some examples of parasites and the diseases they can cause.


This tiny ameba can affect the eye, the skin, and the brain. It exists all over the world in water and soil. Individuals can become infected if they clean contact lenses with tap water.


This disease that comes from parasites that are spread by ticks. It affects the red blood cells. The risk is highest in summer in the Northeast and upper Midwest of the United States.


This is passed on by Balatidium coli, a single-cell parasite that usually infects pigs but can, in rare cases, cause intestinal infection in humans. It can be spread through direct contact with pigs or by drinking contaminated water, usually in tropical regions.


This affects the intestines. The blastocystis enters humans through the fecal-oral route. A person can get it by eating food or drink contaminated with human or animal feces where the parasite is present.


This affects the intestines. Coccidia is passed on through the fecal-oral route. It is found around the world. It can also affect dogs and cats, but these are different kinds. Dogs, cats, and humans cannot normally infect each other.


This is caused by the parasite Entamoeba histolytica. It affects the intestines. It is more likely in tropical regions and in areas with high population density and poor sanitation. It is transmitted through the fecal-oral route.


Giardia, or “beaver fever” affects the lumen of the small intestine. If humans ingest food or water contaminated with feces, dormant cysts may infect the body.

Isosporiasis or cystosporiasis

This disease is caused by the Cystoisospora belli, previously known as Isospora belli. It affects the epithelial cells of the small intestine. It exists worldwide and is both treatable and preventable. It is passed on through the fecal-oral route.


This is a disease that is passed on by parasites of the Leishmania family. It can affect the skin, the viscera, or the mucous membranes of the nose, mouth, and throat. It can be fatal. The parasite is transmitted by types of sandflies.

Primary amoebic meningoencephalitis (PAM)

This is passed on through a free-living ameba known as Naegleria fowleri. It affects the brain and the nervous system, and it is nearly always fatal within 1 to 18 days. It is transmitted through breathing in contaminated soil, swimming pools, and contaminated water, but not from drinking water.


Different types of plasmodium affect the red blood cells. It exists in tropical regions and is transmitted by the Anopheles mosquito.


This is caused by Rhinosporidium seeberi. It mainly affects the mucous of the nose, conjunctiva, and urethra. It is more common in India and Sri Lanka but can occur elsewhere. Polyps result in nasal masses that need to be removed through surgery. Bathing in common ponds can expose the nasal mucous to the parasite.


This is a parasitic pneumonia caused by the parasite Toxoplasma gondii. It affects the liver, heart, eyes and brain. It occurs worldwide. People can become infected after ingesting raw or undercooked pork, lamb, goat, or milk, or though contact with food or soil that is contaminated with cat feces.

A person with a healthy immune system will not usually have symptoms, but it can pose a risk during pregnancy and for those with a weakened immune system.


Also known as “trich” this is a sexually transmitted infection (STI) caused by the parasite Trichomonas vaginalis. It affects the female urogenital tract. It can exist in males, but usually without symptoms.

Trypanomiasis (Sleeping sickness)

This is passed on when the tetse fly transmits a parasite of the Trypanosoma family. It affects the central nervous system, blood, and lymph. It leads to changes in sleep behavior, among other symptoms, and it is considered fatal without treatment. It can cross the placenta and infect a fetus during pregnancy.

Chagas disease

This affects the blood , muscle, nerves, heart, esophagus and colon. It is transmitted through an insect bite. Over 300,000 people in the U.S. have the parasite that can lead to this disease.

Worms, or helminth organisms, can affect humans and animals.

Anisakiasis: This is caused by worms that can invade the intestines or the stomach wall. The worms are passed on through contaminated fresh or undercooked fish and squid.

Roundworm: Ascariasis, or a roundworm infection, does not usually cause symptoms, but the worm may be visible in feces. It enters the body through consuming contaminated food or drink.

Raccoon roundworm: Baylisascaris is passed on through raccoon stools. It can affect the brain, lungs, liver, and intestines. It occurs in North America. People are advised not to keep raccoons as pets for this reason.

Clonorchiasis: Also known as Chinese liver fluke disease, this affects the gall bladder. Humans can become infected after ingesting raw or poorly processed or preserved freshwater fish.

Dioctophyme renalis infection: The giant kidney worm can move through the wall of the stomach to the liver and eventually the kidney. Humans can become infected after eating the eggs of the parasite in raw or undercooked freshwater fish.

Diphyllobothriasis tapeworm: This affects the intestines and blood. Humans can become infected after eating raw fish that live wholly or partly in fresh water. Prevalence has increased in some parts of the developed world, possibly due to the growing popularity of sushi, salted fillets, ceviche, and other raw-fish dishes.

Guinea worm: This affects subcutaneous tissues and muscle and causes blisters and ulcers. The worm may be visible in the blister. As the worms are shed or removed, they enter the soil or water, and are passed on from there.

Hookworm: These can cause intestinal disease. They lay their eggs in soil and the larvae can penetrate the skin of humans. Early symptoms include itching and a rash. They are most common in damp places with poor sanitation.

Hymenolepiasis: Humans can become infected by ingesting material contaminated by rodents, cockroaches, mealworms, and flour beetles.

Echinococcosis tapeworm: Cystic echinococcosis can lead to cysts in the liver and lungs, and alveolar echinococcosis can cause a tumor in the liver. Humans can be infected after eating foods contaminated by the feces of an infected animal, or from direct contact with an animal.

Enterobiasis pinworm: A pinworm, or threadworm, Enterobius vermicularis can live in the colon and rectum of humans. The worm lays eggs around the anus while a person sleeps, leading to itching. It spreads through the oral-fecal route.

Fasciolosis liver fluke: This affects the gall bladder and liver. It is common in countries where cattle or sheep are reared, but rare in the U.S. It can affect the liver and the bile ducts and it causes gastrointestinal symptoms. It passes from one mammal to another through snails. A person may get it from eating watercress, for example.

Fasciolopsiasis intestinal fluke: This affects the intestines. It can also transmitted when consuming contaminated water plants or water.

Gnathostomiasis: This causes swellings under the skin, and occasionally affects the liver, the eyes, and the nervous system. It is rare, but it can be fatal. It occurs in Southeast Asia. It is transmitted by eating freshwater fish, pigs, snails, frogs, and chicken.

Loa loa filariasis: Also known as loaisis, this is caused by the Loa loa worm, or African eye worm. It causes itchy swellings on the body. It occurs mainly in Central and West Africa and is transmitted through deerfly bites.

Mansonellosis: This is passed on through the bites of midges or blackflies. It affects the layers under the surface of the skin, but it can enter the blood. It can lead to angioedema, swellings, skin rash, fever, and joint problems. It is present in Africa and Cental America.

River blindness: Caused by a worm known as Onchocerca volvulus, this affects the eyes , skin, and other body tissues. It is found near fast flowing water. It is transmitted through the bite of a blackfly. It occurs in South America, but 90 percent of cases are in Africa.

Lung fluke: Also known as paragonimiasis, this affects the lungs , causing symptoms similar to those of tuberculosis (TB). However, it can reach the central nervous system, leading to meningitis. It is transmitted when eating undercooked or raw freshwater crabs, crayfishes, and other crustaceans. It is most common in parts of Asia.

Schistosomiasis, bilharzia, or snail fever: There are different types of schistosomiasis. They can affect the skin and internal organs. It results from exposure to fresh water that has snails in it that are infected with the blood fluke, or trematode worm. The worms are not found in the U.S. but they are common worldwide.

Sparganosis: Humans can become infected if they eat foods tainted with dog or cat feces that contains the larvae of a tapeworm of the Spirometra family. It can lead to a migrating abscess under the skin. It is rare.

Strongyloidiasis: This can lead to severe and possibly fatal immunodeficiency. The parasite penetrates through the skin and affects the lungs, skin, and intestines. It is passed on through direct contact with contaminated soil. It most occurs in tropical and subtropical regions.

Beef and pork tapeworms: Taeniasis is caused by tapeworms of the taenia family. They affect the intestines. They are passed on by eating undercooked beef or pork.

Toxocariasis: A roundworm transmits this infection from animals to humans. It affects the eyes, brain, and liver. It is caused by accidentally swallowing the eggs of the parasite, for example, when young children play with soil. Nearly 14 percent of people in the U.S. have antibodies, suggesting that millions have been exposed. Most never have symptoms.

Trichinosis: This is caused by the roundworm of the Trichinella family. Infection can lead to intestinal symptoms, fever, and muscle aches. It is passed on by eating undercooked meat.

Whipworm: Also known as trichuriasis , whipworms live in the large intestine. Eggs are passed in feces. It is common all over the world. Humans can become infected when ingesting the eggs, for example on unwashed fruit or vegetables.

Elephantiasis lymphatic filariasis: This is transmitted through mosquito bites . The adult worms live in the lymph system. Infection can lead to lyphedema and elephantiasis, in which swelling can cause disfigurement and disability. In the Americas, it is passed on by the Culex quinquefasciatus mosquito.

Ringworm is sometimes mistaken for a worm, but it is not a worm. It is a fungal infection.


Bacteria are cellular, prokaryotic and reproduce asexually via binary fission, allowing for the rapid growth under favourable conditions. Many can form endospores which are resistant to extreme conditions, have a thick capsule which helps to overcome the host’s defences and have a cell wall. They cause disease by disrupting the functioning of some cells or organs of the body, producing toxins, interfering with the host’s immune system and disrupting normal signal transduction in the cell. They also have extra chromosomal pieces of circular DNA called plasmids which can be exchanged between bacteria. Some examples of diseases caused by bacteria include tetanus, whooping cough, meningococcal, salmonella and tuberculosis.


The nematode (roundworm) Enterobius vermicularis is widely known as the human pinworm due to the female&rsquos long, pointed tail. In some areas the common names &ldquoseatworm&rdquo and &ldquothreadworm&rdquo are used (the latter of which is sometimes also used to refer to Strongyloides stercoralis). Another putative pinworm species, Enterobius gregorii, has been described and reported from humans in Europe, Africa, and Asia. However, further morphologic and molecular evidence suggests E. gregorii likely represents an immature form of E. vermicularis. The rat pinworm, Syphacia obvelata, has also very rarely been reported infecting humans.

Life Cycle

Gravid adult female Enterobius vermicularis deposit eggs on perianal folds . Infection occurs via self-inoculation (transferring eggs to the mouth with hands that have scratched the perianal area) or through exposure to eggs in the environment (e.g. contaminated surfaces, clothes, bed linens, etc.) . Following ingestion of infective eggs, the larvae hatch in the small intestine and the adults establish themselves in the colon, usually in the cecum . The time interval from ingestion of infective eggs to oviposition by the adult females is about one month. At full maturity adult females measure 8 to 13 mm, and adult males 2 to 5 mm the adult life span is about two months. Gravid females migrate nocturnally outside the anus and oviposit while crawling on the skin of the perianal area . The larvae contained inside the eggs develop (the eggs become infective) in 4 to 6 hours under optimal conditions .

Rarely, eggs may become airborne and be inhaled and swallowed. Retroinfection, or the migration of newly hatched larvae from the anal skin back into the rectum, may occur but the frequency with which this happens is unknown.


Oxyurid nematodes (pinworms) generally exhibit high host specificity. Humans are considered the only host for E. vermicularis, although occasional infections have been reported in captive chimpanzees.

Geographic Distribution

E. vermicularis occurs worldwide, with infections occurring most frequently in school- or preschool-children and in crowded conditions.

Clinical Presentation

Enterobiasis is frequently asymptomatic. The most typical symptom is perianal pruritus, especially at night, which may lead to excoriations and bacterial superinfection. Occasionally, invasion of the female genital tract with vulvovaginitis and pelvic or peritoneal granulomas can occur. Other symptoms include, teeth grinding, enuresia, insomnia, anorexia, irritability, and abdominal pain, which can mimic appendicitis. E. vermicularis larvae are often found within the appendix on appendectomy, but the role of this nematode in appendicitis remains controversial. Very rare instances of eosinophilic colitis associated with E. vermicularis larvae have been reported.

Did God Create Parasites?

This revolting nematode lives in human intestines and can grow up to a foot long and almost one-quarter inch in diameter. The US Centers for Disease Control and Prevention estimates that more than one billion people worldwide are infected with this worm. The scariest fact is you could have one in your small intestine for a long time before you even noticed.

Other roundworms can cause anemia, blindness, or a gross enlargement of the limbs called elephantiasis. Most pet owners treat their animals for a roundworm called heartworm that is spread by a mosquito and can kill dogs. And nematodes infect all sorts of other animals and plants. How did these disgusting parasites become part of God’s “very good” creation?

The simple answer is that God created nematodes during Creation Week as a normal, beneficial part of the environment.

Evolutionists like Carl Zimmer believe roundworms are just part of the dog-eat-dog (or nematode-eat-dog) world that he thinks has persisted for millions of years. In his book Parasite Rex, he mockingly suggests Adam must have been “created” already loaded with parasites. Or, he says, maybe there was “a second creation , an eighth day added on to that first week—‘and on the following Monday God created parasites.’”

By definition, all parasites live at least part of their lives in or on another organism and cause harm. But not all symbiotic relationships are parasitic. And that may hint at an answer to the origin of parasites.

For example, termites have protozoans and bacteria in their guts to help them digest wood. Also, most plants have fungi that help their roots absorb nutrients. These types of relationships are mutualistic (the opposite of parasitic). In many cases, they are so important that one of the organisms couldn’t survive without the other. Humans are an example. We couldn’t live without the trillions of mutualistic microorganisms in our intestines and elsewhere in our bodies.

Roundworms are not all parasitic, either. Some help remove decaying plants and are harmless. Another example of a roundworm with a nonparasitic lifestyle is the threadworm, Strongyloides stercoralis. It lives in the soil, feeding on bacteria and helping the soil ecosystem thrive. However, when living conditions become less favorable, the next generation of threadworm develops into a parasitic stage that lives in human intestines.

God created roundworms as part of his good creation . They lived harmoniously in plants or animals or the soil, helping maintain the balance of nature. Only after the Fall did some roundworms take on a parasitic role, as part of the Curse on creation . So Zimmer was partly right! God included roundworms in his creation .

He may even have created humans already equipped with beneficial roundworms.

Does parasitism, one of Bacteria's lifestyle? - Biology

(1) p. 37-41, Bacteria
The organisms that make up kingdom Monera are all prokaryotic.
These prokaryotic cells are bacteria.
Bacteria is made up of organisms that are one tiny cell each. They are single-cellular. They can only be seen with a microscope.
So if you can actually see any living thing, you will know it is not made of only one cell, but is multi-cellular.

See these images of how prokaryotic cells may be drawn differently.
Image 1, Image 2, Image 3 (scroll down)
There may be more than one correct name of a certain part of a cell. DNA and nucleoid, for example.
Also notice fimbriae (sing. fimbria) and pili. (This is because of which job the fimbriae are doing -- bottom of p. 39)

But the main thing to know is that prokaryotic cells do not have organelles (little organs) like a eukaryotic cell does.
►See this image that compares the two kinds of cells.
--In the cytoplasm (also called cytosol) of a prokaryotic cell, there are ribosomes and DNA.
--In the cytoplasm of a eukaryotic cell, there are many organelles, each with their own job.

►The fimbriea/pili are not used to move the bacterium. They are for grasping. They grasp surfaces to adhere to them (fimbriea), or they grasp other bacteria as part of reproduction (pili).
►Prokaryotic DNA is arranged in a winding, circular shape that connects end-to-end. There is only one replication origin (original DNA strand) when replication starts.
►By contrast, eukaryotic DNA is linear (in a line) it does not connect end to end to form a circle. The DNA in a eukaryotic cell is enclosed in a nucleus -- it is "membrane-bound." Other organelles are enclosed in membranes also, much like little water balloons of all shapes.
In eukaryotic cells, when the DNA is replicated, there are as many as 1000 replication origins.

Despite these differences, however, the underlying process of replication is the same for both prokaryotic and eukaryotic DNA.

►More about: Prokaryotes , Eukaryotes
(when used as an adjective, these words end in -ic)

The shapes of bacteria. There are three basic shapes of bacteria. (see image ), source .
-Sperical (cocci), which is round.
-Rod-shaped (bacilli), which are longer.
-Helical (spirilla), which look like a spiral.
These three shapes of bacteria have variations and different groupings. (see image ) source .

Read about the size of bacteria, a single, prokaryotic cell of the kingdom Monera.

Shape and Movement of Bacteria You may not understand all this, so listen twice! (You will learn on p. 43 about anaerobic bacteria that do not need oxygen.)

Case for a Creator - Bacterial Flagellum

B. burgdorferi products required for mammalian infection

Efforts to identify B. burgdorferi products that play roles in mammalian infection began by serially passaging strains in culture and correlating loss of infectivity with loss of specific plasmids [55, 104�]. As genetic techniques have become available for B. burgdorferi analysis, other approaches have been applied to identifying genes that contribute to mammalian infectivity and the roles of some such genes have been rigorously defined by inactivation and restoration (complementation). The products of these genes can be divided into ones that play physiological roles and those that contribute to survival in the face of other aspects of the host environment, including normal defenses. Since B. burgdorferi has a limited biosynthetic capability, the bacteria rely on their host (or culture medium) for many nutrients that other bacteria can synthesize. Some enzymes shown to be crucial for bacterial survival in a mammalian host (although dispensable for growth in rich culture medium) include PncA, a nicotinamidase involved in production of NAD [109], and two products involved in purine nucleotide synthesis [32, 110]. Another unusual feature of B. burgdorferi is that it does not contain intracellular iron and, hence, does not use iron as a co-factor for enzymes [111], thereby facilitating survival in the iron-poor mammalian environment.

Factors important for bacterial survival in a mammalian host can be subdivided into two groups. Some are involved in early infection, such as OspC [112�], which are presumably required for host colonization or resistance to innate immunity. Others are involved in resistance to acquired immunity, such as VlsE [13, 109, 117, 118].

The OspC product has been shown to be required for an early stage of mammalian infection [113�] and conflicting data have been presented regarding its importance in tick transmission [113, 114, 119, 120]. OspC production begins in feeding ticks (or immediately after needle inoculation, Fig. 1 ) and lasts for the first couple of weeks of mammalian infection [49, 51, 103, 121�]. Once the bacteria are established in a host, OspC production is not required for persistence [114]. The molecular function of the OspC protein is undefined, although the crystal structure of the protein predicts that it binds a small ligand [124, 125]. The gene product contains a highly variable region that has allowed characterization of OspC types [126�]. Studies of the significance of OspC type have reached contradictory conclusions. Some studies have found a correlation between OspC type and wild rodent host specificity [129], but others found no such link [130]. The OspC types of human clinical isolates have also been correlated with invasive and non-invasive phenotypes [128, 131]. However, similar studies demonstrated greater diversity among invasive types than previously recognized, calling into question any causative effect of OspC sequence on invasion [130, 132]. Carefully controlling for variables other than OspC has not been possible in such studies, since additional genome components of these isolates also differ. Recently, the invasive OspC protein types were shown to bind plasminogen [131], a trait shared with other B. burgdorferi proteins [133, 134] that was suggested to facilitate B. burgdorferi infection of mammals and ticks [135].

Considerable attention has been paid to several B. burgdorferi proteins that co-opt the complement regulators factor H and factor H-like proteins, which normally protect mammalian cells from attack by their own complement by blocking activation of the alternative pathway [136�]. Surface coating with factor H could protect the bacteria from killing or opsonization by host complement, facilitating infection. Surprisingly, however, B. burgdorferi infection of mice deficient in factor H production did not differ from infection of wild type mice, suggesting that any protection conferred by factor H binding was unimportant or redundant [85]. Because of these findings, the significance of the factor H-binding proteins for B. burgdorferi remains unknown.

VlsE is a B. burgdorferi protein that is required for persistent mammalian infection [13, 109, 117, 118] and whose synthesis begins around the time that OspC production ceases ( Fig. 1 )[103]. Although its function is unknown, this lipoprotein has an elaborate system for variation [22]. Variation at the VlsE locus appears to be required for persistence [118, 142], probably because its (unknown) essential function requires its presence on the bacterial surface, where it will be targeted by the adaptive immune response of the mammalian host.

Since B. burgdorferi can survive in the face of a neutralizing antibody response, the idea that the bacteria shelter in tissues with little exposure to antibodies by interacting with the host extracellular matrix (ECM) has been investigated (reviewed by Cabello et al. [143] and Coburn et al.[144]). Among the B. burgdorferi proteins that bind ECM components are DbpA and DbpB, which bind decorin [145], BBK32, which binds fibronectin [146], Bgp, which binds proteoglycans [147], and P66, which binds integrins [148]. These proteins may help B. burgdorferi migrate through mammalian tissue and persist in joints and skin, where the spirochetes may be inaccessible to circulating antibodies. Genetic studies, however, have shown that spirochetes lacking DbpA have little if any defect in mammalian infectivity [149]. Two studies of mutants lacking BBK32 also found a small [150] or no [151] decrease in infectivity. These studies call into question whether interaction with host ECM protects the bacteria. Alternatively, the multiplicity of ECM-binding proteins may ensure such interaction by providing redundant binding proteins so that the loss of any single protein may fail to yield a noticeable phenotype.

An accumulating body of evidence demonstrates that a cascade of transcriptional regulators, encoded by the rpoS and rpoN genes, controls the production of a number of lipoproteins in response to changing environmental factors [152, 153]. A sensor-regulator pair of proteins also contributes to this regulation [154]. When B. burgdorferi mutants were tested in the infection model, rpoS and rpoN expression were found to be required for infecting mice [153, 155], demonstrating the importance of this pathway for survival in a mammal.


A second type of symbiotic relationship is called mutualism, where two species benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut (Figure 2a). The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood chewing actions of the termite. Lichens have a mutualistic relationship between fungus and photosynthetic algae or bacteria (Figure 2b). As these symbionts grow together, the glucose produced by the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae.

Figure 2. (a) Termites form a mutualistic relationship with symbiotic protozoa in their guts, which allow both organisms to obtain energy from the cellulose the termite consumes. (b) Lichen is a fungus that has symbiotic photosynthetic algae living inside its cells. (credit a: modification of work by Scott Bauer, USDA credit b: modification of work by Cory Zanker)

Mutualism, Commensalism, and Parasitism

Hi, and welcome to this video on mutualism, commensalism, and parasitism!

Have you ever noticed any interactions between organisms in nature that you thought were especially interesting? Like bees pollinating flowers or clownfish living in sea anemones? Symbiotic relationships like these are all around you if you know where to look. Organisms can use such a relationship to benefit from one another in several ways, such as transportation, food, shelter, growth, and reproduction, just to name a few. So if we take the two examples we just mentioned, the bees pollinating the flowers and the clownfish living in sea anemones, we have two classic examples of how organisms can mutually benefit from one another so that both organisms can thrive. When both organisms in a symbiotic relationship benefit, we call this mutualism. In the case of the bees and the flowers, bees need pollen to make honey which they use as a food source, so the bees go from flower to flower gathering pollen which they store in a pouch in their abdomen or on their hind legs depending on the species. When the bees move on from one flower to the next, some of the pollen brushes off and pollinates the new flower. Both the bees and the flowers benefit from this relationship, so it’s a good example of mutualism. Clownfish and sea anemones have the same sort of mutualistic relationship. To other fish, brushing up against a sea anemone is deadly. But clownfish are unaffected by the anemone’s sting because they have adapted to form a protective mucous on their skin. So the clownfish is able to live in the sea anemone and in the process keeps it clean, while the sea anemone gives the clownfish protection and a place to live.

Another example of mutualism that you may not have thought of is the symbiotic relationship between, us, humans, and the bacteria in our gut. Take lactobacillus bacteria for a specific example. Lactobacilli are a common type of bacteria found in yogurt, cheese, and some plants. So when you eat any of these foods, the bacteria will make a home out of your intestines by feeding off of the sugars you eat while simultaneously helping you digest that sugar. Both parties benefit, so this is also a mutualistic relationship.

Commensalism is another type of symbiotic relationship where one organism benefits and the other organism isn’t benefited or harmed either way. Golden jackals will follow tigers on their hunt for prey so that they can feed off of the tiger’s scraps. The tiger does all of the work to actually catch and kill its prey, but it doesn’t seem to mind the jackal cleaning up after it. Since the jackal benefits and the tiger isn’t affected, we can say that this is an example of commensalism. Another example of commensalism is one organism using another as a means of transportation. A lot of insects, fish, and other animals use each other in this way, but a good example is the remora. This is a type of suckerfish that will attach itself to sharks and other big fish to catch an underwater ride. This in and of itself is an example of commensalism since only the remora really benefits, but this relationship can change to mutualism when the remora feed on the parasites on the backs of these big fish. This leads us to our last type of relationship, which is parasitism.

Parasitism is a type of relationship where one organism benefits and the other organism is harmed in some way. Your mind might jump to what we more commonly think of as a parasite like tapeworms or fleas. These are great examples because in both cases, the parasite benefits while the other organism is harmed. As humans, we can get tapeworms from the food and water we consume if it is not treated or prepared properly. Once the tapeworm is inside of the digestive tract, it eats a lot of your food for you. So symptoms can range from increased appetite to nausea, but if the tapeworm spreads to other organs it can be life-threatening. However, parasitic relationships aren’t limited to the microscopic or small-scale world. Cowbirds are a species of birds that instead of raising their own young, take advantage of another bird species, since birds cannot easily distinguish between their young. Female cowbirds will lay their eggs in another bird’s nest like a black-capped chickadee, and the female black-capped chickadee will feed both her own young and the cowbird nestling. However, cowbirds are much larger than most birds so they will demand more of the food and nest space. In the end, this means some of the black-capped chickadee’s young will die while the cowbird nestling lives.

So, to review, mutualism is where both organisms benefit, commensalism is where one benefits and the other is unaffected, and parasitism is where one benefits and the other is harmed.

Before we go, here’s a review question:

Which is the best example of mutualism?

  1. A flea and a dog
  2. A squid and an anglerfish
  3. Cattle and crows
  4. A poison dart frog and a cricket

The answer is C. Crows and other birds will get a free meal by eating insects and fleas off of the backs of cattle and cattle will get a free cleaning. Both the crows and the cattle benefit, so that makes this relationship mutual.

Additional file 1: Table S1. Accession numbers of genes used in the phylogenetic analysis. (DOC 58 KB)


Additional file 2: Figure S1 Phylogenetic trees of each of the individual genes used in the study. Posterior probabilities are given above the node and maximum likelihood values are given below. Branch lengths are indicated by the scale bar of substitutions per site at the bottom left of each gene tree. (JPEG 779 KB)


Additional file 3: Figure S2 Split networks for each of the individual genes used in the study. A test of tree-likeness was carried out on each of the individual gene and only the 95% confidence network is shown, indicating only the statistically significant splits. Branch lengths are indicated by the scale bar of substitutions per site at the bottom left of each gene tree. (JPEG 697 KB)

Table S2.

Additional file 4: The distribution of Rickettsia among arthropods. Incidence data is given for the unpublished wasp and worldwide screen. (DOC 2 MB)