Types of Microbial Interactions


Microorganism commonly interacts among each other in a positive or negative manner, neutralism is a rare and perhaps only theoretical phenomenon. Within a single population, microbes cooperate at low densities and at competes at high cell densities. Each group of the organism has had to adapt itself during its evolution not only to the non-living environment but also to other organisms that surrounded it. Varieties of way are adapted by microbes to tolerate physical environment, similarly, microbes interact with other microbes can make an association with the plant, animal, or other microbes.

  • Microorganisms interact with each other and can be physically associated with other organisms in a variety of ways.
  • One organism can be located on the surface of another organism as an ectobiont or located within another organism as endobiont.

Microbial interaction may be positive such as mutualism, proto-cooperation, commensalism or can be negative such as parasitism, predation or competition

A) Positive interactions

a) Mutualism

Mutualism is an obligatory relationship between two populations that benefits both populations. A mutualistic relationship is highly specific β€” one member of the association ordinarily cannot be replaced by another related species. Mutualism requires close physical proximity between the interacting populations. Relationship of mutualism allows organisms to exist in a habitat that could not be occupied by either population alone. The metabolic activities and physiological tolerance of the populations involved in the mutualistic relationship are normally quite different from those of either population by itself.

Example β€” The relationship between certain algae or cyanobacteria and fungi that result in the formation of lichens are probably the most outstanding


Features of mutualisms :

  • It is a type of endosymbiosis.
  • It is an obligatory interaction as both partners do not survive individually.
  • It is beneficial for both partners.
  • It is a bidirectional phenomenon where both the partners are dependent on each other.

b) Commensalism

A type of symbiosis in which one organism is benefited, while other remains unaffected. It is a unidirectional relationship between two populations: the unaffected population by definition does not benefit from, nor it is negatively affected by the action of the second population. There are many physical and chemical basis of commensalism. The basis of commensalism is

  • Modification of habitat

An unaffected population in the course of its normal growth modifies the habitat in such a way that it is more suitable for another population.

  • Conversion of the complex substrate into simpler for the growth of another population

When one population results produce an oxidation product which is used for the growth and metabolism of another population e.g. Mycobacterium oxidizes cyclohexane to cyclohexanol. Cyclohexanol is utilized for the growth of other populations.

  • Removal or neutralization of toxic material

The oxidation of H2S by Beggiatoa is an example of detoxification that benefits from its relationship with the second population. Precipitation of mercury by sulfate reducers is an example of detoxification. The neutralization or detoxification of toxic material provides nontoxic substrates for the growth of another population.


1) Association between Flavobacterium brevis and Legionella pneumophilia – This association is based on the production of growth factors. Flavobacterium excretes cysteine, which is used by Legionellain its aquatic habitat.

2) Association between Desulfovibrio and Methano bacterium β€” Desulfovibrio ferments sulfate and lactate to produce acetate and hydrogen which are utilized by Methanobacterium.

Features of commensalisms

  • It is a type of ectosymbiosis.
  • It is not an obligatory interaction.
  • It is beneficial for one of the partners
  • It is a unidirectional phenomenon.

c) Syntrophy

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 benefits.
  • 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 the cooperation of population 2. Population 2 is 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

i) The methanogenic ecosystem in a sludge digester

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

ii) 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.

e) Neutral association or Neutralism :

Neutralism describes the relationship between two species which do interact but do not affect each other. It is to describe interactions where the fitness of one species has absolutely no effect whatsoever on that of other. True neutralism is extremely unlikely and impossible to prove. When dealing with the complex networks of interactions presented by ecosystems, one cannot assert positively that there is absolutely no competition between or benefit to either species. Since true neutralism is rare or nonexistent, its usage is often extended to situations where interactions are merely insignificant or negligible.

f) Symbiosis

The close association between two different organisms is called symbiosis or microbial interaction.

Importance of symbiosis

To determine function or importance fulfilled by a partner in symbiosis it is necessary in all but the most obvious cases to separate the partners and study their requirements in the association. In many cases, this has not yet been achieved either because the symbiont cannot separate without damaging them or because an isolated partner cannot be cultivated.

Functions of symbiosis are as follows :

1) Protection – Endosymbionts, as well as ectosymbionts that live in the body cavities of animals, are protected from any adverse environmental conditions. These habitats protect the symbiont from desiccation.

2) Provision for the favorable position – A symbiotic association may provide one partner with a position that is favorable with respect to the supply of nutrient e.g. many marine invertebrates for their photosynthetic algal symbionts.

3) Provision for recognition of devices

4) Nutrition – The common function of symbiosis is providing nutrients for their partners. The provision of nutrients may be indirect, as in the case of fungal that infect plant roots and thereby increase the water-absorbing capacity of the root system. A most dramatic and extensively studied example is nitrogen fixation.

B) Negative interactions

a) Prey

Bacteria are constantly exposed to a multitude of threats: bacteriophages can infect and kill bacteria: amoebae, nematodes, and insects can prey on prokaryotes, and competitor strains fight for the same resources. In order to survive in this battlefield, bacteria have evolved highly effective defense mechanisms. Because killing and deterring the antagonists are powerful ways to thrive in this environment, bacteria display a great diversity of toxins and antibiotics that selectively act on their enemies. Amoebae are voracious and ubiquitous predators to bacteria that cause constant depletion of huge bacterial reservoirs. This puts both organisms under strong evolutionary selection pressure: the bacteria have evolved mechanisms to prevent grazing and the amoebae must counteract or surmount these mechanisms in order to survive.

b) Amensalism (Antagonism)

The association in which one organism secretes a substance that is toxic to another organism, such interaction is known as amensalism or antagonism. Microorganisms that are capable of producing toxic substances have a competitive advantage over the other population. Once the competitive organism establishes itself in the habitat it prevents the growth of another organism


1) The production of lactic acid by lactic acid bacteria prevents the growth of other organisms in that habitat.

2) In the rumen anaerobic heterophilic microbial population produces volatile fatty acid which prevents the colonization of E.co/i in the rumen.

3) Acids produced by microbial populations in the vaginal tract are responsible for preventing the growth and hence infection by pathogens such as Candida albicans

4) Oxidation of S2 by Thiobacillus thioxidans produces sulfuric acid which lowers the pH (1-2) of the surrounding this precludes the possibility of growth of other organisms

5) Production of O2 by algae preclude the possibility of growth of obligate aerobes

6) Some microorganism produces antibiotics which are inhibitory for the growth of other organisms in the surrounding.

Features of Amensalism

  • It is a type of ectosymbiosis.
  • It is beneficial for one of the partner but harmful for other
  • Occasionally, death of host may occur.
  • It’s a unidirectional phenomenon.

c) Antibiosis

Antibiosis the production of inhibitory compounds called antibiotics is actually a form of antagonism. Hundreds of naturally occurring antibiotics have been isolated from bacteria and fungi and used as drugs to control diseases. Bacteriocins are another class of antimicrobial proteins that are toxic to bacteria other than the ones that produced them.

d) 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

Competition between Paramecium cadatum 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. caudarum due to the competition.

e) 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 endo-parasite.

Examples of parasitism

  • Viruses- Viruses are an obligate intracellular parasite that exhibits great host specificity. There are many viruses that are a parasite to bacteria (bacteriophage), fungi, algae, protozoa, etc.
  • Bdellovibrio- Bdellavibrio is ectoparasite to many Gram-negative bacteria. The parasite Bdellovibrio penetrates the outer membrane of its host and enters periplasmic space but not inside host cytoplasm.

f) Predation

The interaction in which one organism eats or engulfs other organism is called predation. The organism being engulfed or eaten is called prey and the organism eating or engulfing is called predator. The interaction continues until the population of prey exists, the decline in the population of prey will limit the food/nutrients for predators and as a result, the population of predators will also decrease. When Didiniumnasutum is introduced with Paramecium caudatum, it preys on Paramecium until the paramecium population becomes extinct. Extinction of Paramecium causes loss of food for Didinium and hence Didiniim population also becomes extinct. If few members of Paramecium population can hide and escape the predation by Didinitum then the Paramecium population can recover after the extinction of Didinium.


  • Association between Didinium nasutumi (predator) and Paramecium caudatum (prey).
  • Paramecium bursaria (predator) and Schizosaccharomyces pombe (prey)
  • Tetrahymena pyriformis (predator) Klebsiella pneumoniae (prey)

Features of predation

  • It is a type of ecto-symbiosis.
  • In this interaction death of prey occurs.
  • It is beneficial for one of the partner but destroys other
  • It is a unidirectional phenomenon.