HOST-PARASITE RELATIONSHIPS (MICROBIAL ASSOCIATIONS)

A host-parasite relationship is an association that exists between two organisms known as the host and the parasite, in which both organisms either derive benefit from the relationship or is harmed in the process. Microorganisms are ubiquitous, and they often exist in association with other forms of life in their ecological niches including man, plants animals and other microbes. The human host for example is regularly in contact with microorganisms. However, only a few of these microbes are able to establish themselves within the host tissues and/or cells to either cause disease or improve the host’s health. To survive and reproduce, the host and the parasite must co-evolve in such a way that their association does not leave any untoward effect on each other. Nevertheless, some host-parasite relationship such as parasitism is only beneficial to the parasite while the host suffers or dies in the association.

The evolution of life on planet earth has allowed both the host and its parasite to co-evolve in several beneficial and disadvantageous relationships. And to survive, the host continually develops novel strategies or mechanisms that protect it from the detrimental activities of the parasite. The co-evolution of the host and the parasite has also allowed the parasite to also develop mechanisms that enable it to survive within the host. Microorganisms in the soil, water, air and in other parts of the environment exhibit or go into a host-parasite association for several reasons including but not limited to the derivation of nutrients, production  of important metabolites or for protection. Bacteria for example are consistently associated with the body surfaces of animals, man and plants. And in the soil, nitrogen-fixing bacteria help in nitrogen fixation – an important growth process required by plants especially leguminous plants.

There are many more bacterial cells on the surface of humans (including the gastrointestinal tract) than there are human cells that make up the body; and these organisms go into association with their human host to either harm them or derive benefit from them. The bacteria and other microbes that are consistently associated with an animal are called the indigenous microbiota or normal microflora of the animal or human. And these microorganisms exhibit a variety of symbiotic interactions including parasitism, commensalism, amensalism, competition, predation, cooperation and mutualism with their animal or human hosts. Amensalism, predation, competition and parasitism are harmful interactions exhibited by microbes while commensalism, cooperation and mutualism are beneficial interactions that microorganisms exhibit in their niche habitat.

SYMBIOSIS

A symbiotic association is a relationship that exists between two or more different organisms. This type of association is often close, and it usually last for a long period of time. Microorganisms and non-microbial organisms can go into a symbiotic association for diverse reasons which can either be beneficial or harmful to any of the organism in the relationship. The symbiotic association of microorganisms (bacteria in particular) with other forms of living organisms is exemplified in various activities of the ecosystem. For example, some bacteria known as nitrogen fixing bacteria (e.g. Rhizobium species) fix nitrogen in symbiotic association in leguminous plants (e.g. beans and peas). This ability of Rhizobium species and other nitrogen fixing bacteria to convert atmospheric nitrogen (N₂) to ammonia (NH₃) through the process of nitrogen fixation is a critical biological process in the ecosystem because it readily makes N₂ to be available in the form in which plants and animals can utilize it.

Some microorganisms also go into symbiotic associations with other higher organisms such including but not limited to animals, insects, plants and other invertebrates, where they provide a number of beneficial effects to their partners. Bacteria in the stomach of ruminant animals (e.g. cows) help the animals to digest the cellulose component of the plant they feed on, and in the process provide vitamins, proteins and carbon for the ruminant. Beneficial microorganisms (inclusive of bacteria and some fungi) also live in close association with humans, and are found on the skin and in the hosts body where they provide a number of protection against pathogenic organisms as well as help to synthesize vitamins and some growth promoting factors in the body. Symbiotic association includes mutualism, parasitism, competition, predation, cooperation and commensalism.   

MUTUALISM

Mutualism is a type of association in which both organisms in the relationship benefits from each other. These organisms are generally known as symbionts. The symbiotic association of fungi and cyanobacteria or algae to form lichens is a typical example of a mutualistic microbial relationship. In a mutualistic associations, two organisms of different species work together, and each benefit from the relationship. Organisms in a mutualistic relationship evolved together. Each of the organisms in mutualistic relationship was part of the other’s environment, so as they adapted to their environment, they made use of each other in a way that benefited both. Mutualism is exhibited by the beneficial bacteria in the gut or gastrointestinal tract of either humans or animals. The beneficial bacteria found in the human or animal gut is also an example of a mutualistic relationship. While the human’s gastrointestinal tract (GIT) or gut cannot digest all of the food that it eats, the beneficial bacteria in the human gut eat the food that the human cannot digest (by breaking it down) and partially digest it, thus allowing the human to finish the job of digestion. The beneficial bacteria benefit by getting food, and the human benefits by being able to digest the food it eats. Mutualism is generally a symbiosis in which both members benefit from the relationship.

PARASITISM

Parasitism is a type of relationship in which one partner (known as the parasite)benefits at the expense of the other partner (known as the host) in the association. The term parasite refers to an organism that grows, feeds and is sheltered on or in a different organism while contributing nothing to the survival of its host. Parasitism is a non-mutual symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host. A parasitic relationship is one in which one organism, the parasite, lives off of another organism, the host, harming it and possibly causing death.

The parasite lives on or in the body of the host. It is this type of relationship that leads to the establishment of disease or infection in human or animal host. In most of the cases, the host can survive from the infection or disease by using its immune system to restrain the untoward effects of the parasite. But in some cases, the host can die as a result of the deleterious activities of the parasite in the host. Not all parasites have to cause disease. Some parasites such as louse, ticks, fleas, and leeches are parasitic insects or arthropods that do not usually cause disease directly, but they do suck blood from their host including animals and humans. These parasites cause some harm and discomfort to their host during the blood meal.

Parasites can also act as vectors (i.e. organisms that transmit disease-causing pathogens to other species of animals and man). Mosquitoes especially the female Anopheles mosquito that harbours the Plasmodium parasites that cause malaria in man are typical examples of vectors because they transmit disease-causing pathogens to their host. The bacteria that cause the bubonic plague (i.e. Yersinia pestis) are carried by rodents, such as rats. The plague bacteria then infect fleas that bite the rats. Infected fleas transmit the bacteria to other animals they bite, including humans. In this case, both the flea and the bacteria are parasites, and the flea is also a vector that transmits the disease causing bacteria (Y. pestis) from the rat to the human host.

COMMENSALISM

Commensalism is an association in which one organism (known as the commensal organism) benefits from the relationship but the other organism neither benefit nor surfer from the alliance. It is symbiotic relationship between two organisms of different species in which one derives some benefit while the other is unaffected. Commensalism is a relationship in which one member benefits, and the other one neither benefits nor is harmed. It is a type ofrelationships between two organisms where one organism benefits from the other without affecting it.In commensalism, one organism in the association benefits and the other derives neither benefit nor harm from the relationship.

Cooperation is a type of symbiosis that benefits both organisms in the relationship. But unlike commensalism, cooperation is not an obligatory type of relationship. In the fungal-plant root interactions (mycorrhizal) for example, normal chlorophyll-containing plants can grow without the help of the fungus; and this shows that the relationship is not obligatory – since some plant can even grow without the support of the fungus. And some fungi, in contrast, do not survive without forming an association with the plant root and vice-versa. 

AMENSALISM

Amensalism is defined as a relationship in which the product of one microorganism or organism has a negative effect on the survival of another organism. It is a harmful type of association like parasitism in which one partner in the relationship suffers the adverse effect of another organism. Amensalism is an adverse microbe-microbe interaction. In amensalism, one organism produces a substance or compound that has inhibitory effect on the growth of another organism. The secreted compound can even have the ability to kill the other microorganism in the relationship.

The phenomenon of amensalism is not only exhibited by microbes. It can also be seen in humans, animals and insects. The molecules secreted by animals and humans as part of their innate immunity (such as those secreted by the phagocytes) have amensalistic effect on the pathogens that invaded the animal or human host; and the human skin also produces substances that limit the growth of pathogenic organisms on the human body. Some microbial products especially those with antimicrobial effect (e.g. antibiotics) can produce amensalistic effects on the organisms they are in a relationship with; and this could lead to the death or inhibition of the growth of the other microbe.

COMPETITION

Competition is defined as a microbial interaction between two microbes that are attempting to use the same resources in a given habitat. It arises when two organisms living in a particular population tries to acquire the same resources or nutrients especially those resources that are in short supply in their community. Microbes compete for several factors including but not limited to space, water and nutrients in their environment. And in most of the cases, it is the survival of the fittest that allows the stronger organism to thrive over the less-stronger organism – since the resource(s) they are competing for may be limiting in their environment. The most dominating microbe outcompetes the other organism (the slow-growing microbe); and thus outgrows it since it is now in charge of the limiting nutrient or resource(s) in that particular microbial community.        

PREDATION

Predation is defined as the phenomenon in which the predator attacks and kills the prey.In predation, the prey is usually smaller and physically less-powerful than the predator; and this natural advantage gives the predator an edge over its prey – which it kills and feeds upon. Though predation may be perceived as a harmful association (one that often leads to the death of the prey), it has several beneficial effects. For example, some predators only ingest their prey to give them protection and thus provide a high supply of nutrients to the ingested prey. Ciliates are protozoan’s that ingest the bacterium, Legionella pneumophila in aquatic environments; and thus protect the bacterium from the harmful effects of chlorine – which is often used to control the bacterium in air-conditioning units and cooling towers.      

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