Introductions

• Biological interactions are the effects that the organisms in a community have on one another.
• There are completely different kinds of microbial interactions which incorporates interaction with different microbes, Plant-Germ interactions promoting plant growth, interaction with animals, interaction with humans, and interaction with water, etc.
• Microbial interactions are ubiquitous, diverse, critically important in the function of any biological community, and are crucial in global biogeochemistry.
• The most common cooperative interactions seen in microbial systems are mutually beneficial. The interactions between the two populations are classified according to whether both populations and one of them benefit from the associations, or one or both populations are negatively affected.
• There are many sorts of symbiotic relationships such as mutualism, parasitism, amensalism, commensalism and competition, predation, protocooperation between the organisms.

Types of Microbial Interaction

• Positive interaction: Mutualism, Syntrophism, Proto-cooperation, Commensalism
• Negative interaction: Ammensalism (antagonism), parasitism, predation, competition

Mutualism

• It is defined as the relationship in which each organism in interaction gets benefits from the association.
• It is an obligatory relationship in which mutualist and host are metabolically dependent on each other.
• A mutualistic relationship is very specific where one member of the association cannot be replaced by another species.
• Mutualism requires close physical contact between interacting organisms.
• The relationship of mutualism allows organisms to exist in a habitat that could not be occupied by either species alone.
• The mutualistic relationship between organisms allows them to act as a single organism.

Examples of mutualism:

Lichens: Lichens are an excellent example of mutualism. They are the association of specific fungi and certain genus of algae. In lichen, the fungal partner is called mycobiont and algal partner is called phycobiont is a member of cyanobacteria and green algae (Trabauxua).

Syntrophism

• It is an association in which the growth of one organism either depends on or improved by the substrate provided by another organism.
• In syntrophism, both organisms in association get to benefit from each other.

Compound A
Utilized by population 1
Compound B
Utilized by population 2
Compound C
utilized by both Population 1+2
Products

In this theoretical example of syntrophism, population 1 is able to utilize and metabolize compound A, forming compound B but cannot metabolize beyond compound B without co-operation of population 2. Population 2is unable to utilize compound A but it can metabolize compound B forming compound C. Then both populations 1 and 2 are able to carry out a metabolic reaction which leads to the formation of the end product that neither population could produce alone.

Examples of syntrophism:   

Methanogenic ecosystem in a sludge digester

• Methane produced by methanogenic bacteria depends upon interspecies hydrogen transfer by other fermentative bacteria.
• Anaerobic fermentative bacteria generate CO2 and H2 utilizing carbohydrates which are then utilized by methanogenic bacteria (Methanobacter) to produce methane.

Lactobacillus arobinosus and Enterococcus faecalis

• In the minimal media, Lactobacillus arobinosus and Enterococcus faecalis are able to grow together but not alone.
• The synergistic relationship between E. faecalis and L. arobinosus occurs in which E. faecalis require folic acid which is produced by L. arobinosus and in turn, lactobacillus require phenylalanine which is produced by Enterococcus faecalis.

Protocooperation

• It is a relationship in which an organism in an association is mutually benefited with each other.
• This interaction is similar to mutualism but the relationships between the organisms in protocooperation are not obligatory as in mutualism.

Examples of Protocooperation:

a. Association of Desulfovibrio and Chromatium: It is a protocooperation between the carbon cycle and the sulfur cycle.

b. Interaction between N2-fixing bacteria and cellulolytic bacteria such as Cellulomonas.

Commensalism

• It is a relationship in which one organism (commensal) in the association is benefited while another organism (host) of the association is neither benefited nor harmed.
• It is a unidirectional association and if the commensal is separated from the host, it can survive.

Examples of commensalism:

a. Non-pathogenic E. coli in the intestinal tract of humans: E. coli is a facultative anaerobe that uses oxygen and lowers the O2 concentration in the gut which creates a suitable environment for obligate anaerobes such as Bacteroides. E. coli is a host which remains unaffected by Bacteroides.

b. Flavobacterium (host) and Legionella pneumophila (commensal): Flavobacterium excretes cystine which is used by Legionella pneumophila and survives in the aquatic habitat.

c. Association of Nitrosomonas (host) and Nitrobacter (commensal) in Nitrification: Nitrosomonas oxidize Ammonia into Nitrite and finally, Nitrobacter uses nitrite to obtain energy and oxidize it into Nitrate.

Amensalism (antagonism)

• When one microbial population produces substances that are inhibitory to other microbial population then this interpopulation relationship is known as Ammensalism or Antagonism.
• It is a negative relationship.
• The first population which produces inhibitory substances are unaffected or may gain competition and survive in the habitat while other populations get inhibited. This chemical inhibition is known as antibiosis.

Examples of the antagonism (amensalism):

a. Lactic acid produced by lactic acid bacteria in the vaginal tract: Lactic acid produced by many normal floras in the vaginal tract is inhibitory to many pathogenic organisms such as Candida albicans.

b. Skin normal flora: Fatty acid produced by skin flora inhibits many pathogenic bacteria in the skin

c. Thiobacillus thiooxidant: Thiobacillus thioxidant produces sulfuric acid by oxidation of sulfur which is responsible for lowering pH in the culture media which inhibits the growth of most other bacteria.

Competition

• The competition represents a negative relationship between two microbial populations in which both the population are adversely affected with respect to their survival and growth.
• Competition occurs when both populations use the same resources such as the same space or same nutrition, so, the microbial population achieves lower maximum density or growth rate.
• Microbial population competes for any growth-limiting resources such as carbon source, nitrogen source, phosphorus, vitamins, growth factors etc.
• Competition inhibits both populations from occupying exactly the same ecological niche because one will win the competition and the other one is eliminated.

Examples of competition:

a. Competition between Paramecium caudatum and Paramecium aurelia:  Both species of Paramecium feeds on the same bacteria population when these protozoa are placed together. P. aurelia grow at a better rate than P. caudatum due to competition.

Parasitism

• It is a relationship in which one population (parasite) get benefited and derive its nutrition from other population (host) in the association which is harmed.
• The host-parasite relationship is characterized by a relatively long period of contact which may be physical or metabolic.
• Some parasite lives outside the host cell, known as ectoparasite while other parasite lives inside the host cell, known as endoparasite.

Examples of parasitism:

a. Viruses: Viruses are an obligate intracellular parasite that exhibits great host specificity. There are many viruses that are parasite to bacteria (bacteriophage), fungi, algae, protozoa etc.

b. Bdellovibrio: Bdellavibrio is ectoparasite to many gram-negative bacteria.

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Predation

• It is a widespread phenomenon when one organism (predator) engulf or attack other organisms (prey).
• The prey can be larger or smaller than the predator and this normally results in the death of the prey.
• Normally predator-prey interaction is of short duration.

Examples of Predation:

a. Protozoan-bacteria in soil: Many protozoans can feed on various bacterial population which helps to maintain the count of soil bacteria at optimum level

b. Bdellovibrio, Vamparococcus, Daptobacter, etc are examples of predator bacteria that can feed on a wide range of the bacterial population.