Microbial Interactions

The population of two species may interact for neutral, positive and negative represented as 00, – -, + +, + 0, – 0 and + – . There are mainly nine types of interactions based on whether species are not gaining, gaining or harming to the other species in a given relationship.

 Microorganisms can be physically associated with other organisms in a number of ways. Physical associations can be intermittent and cyclic or permanent

  • Ectosymbiosis- A smaller microorganism remain located on the surface of the larger organism, as an ectosymbiont
  • Endosymbiosis-microorganism is found within the other organism. This phenomenon is known as Endosymbiosis and the organism which is located inside is known as endosymbiont.
  • Ecto/endosymbiosis-In this phenomenon, microorganism lives on both the inside and the outside of the other organism. For example, mycorrhizal  fungi associated with plant roots  often contain endosymbiotic bacteria, as well as have  bacteria living on their surfaces


Competition is a type of interaction, which occurs between two organisms that are using the same limited resource. This leads to reduction in fitness of one or both organism. It can be divided into two categories- intraspecific and interspecific.

  • Interspecific and Intraspecific competition Intraspecific competition is within the same species and interspecific is between different species. Intraspecific competition is more acute as compared to interspecific competition, e.g.,Tigers and leopards compete for the same prey as both are carnivorous animals.
  • Competitive Exclusion principle This principle states that if two species with the same niche coexist in the same ecosystem then one will be excluded from the niche due to intense competition. This principle is also known as Gause’s Hypothesis.


The association between organisms of two different species, in which each individual derives benefit from the activity of other individual is known as mutualism

  • An obligatory association that provides some reciprocal benefit to both partners (some examples are given below)
  • Protozoan-termite relationship-protozoa live in the guts of insects that ingest but cannot metabolize cellulose; the protozoa secrete cellulases, which metabolize cellulose, releasing nutrients that the insects can use
  • Lichens-an association between a fungus (ascomycetes) and an alga (green algae) or cyanobacterium
  • Fungal partner (mycobiont) obtains nutrients from alga by hyphal projections (haustoria) that penetrate the algal cell wall as well as oxygen for respiration
  • Zooxanthellae-algae harbored by marine invertebrates; reef-building (hermatypic) corals use zooxanthellae to satisfy most of their energy needs; the coral pigments protect the algae from ultraviolet radiation
  • Microorganism-insect mutualisms-bacterial endosymbionts provide essential vitamins and amino acids to host insects; insect provides a secure physical habitat and ample nutrients to the bacteria
  • The rumen ecosystem-bacteria in the rumen anaerobically metabolize cellulose to smaller molecules that can be digested by the ruminant; microorganisms produce the majority of vitamins that are needed by the ruminant; methane is also produced in the process
  • Methanogens consume the hydrogen gas during methanogenesis; this promotes further production of fatty acids and hydrogen gas


This is type of interaction involving a parasite and a host. In this interaction, parasite obtain its food directly from a host. In this, there is no immediate death observed of the host as in case of predation.

Types of parasites

  • Temporary parasites These are the parasites which attach to host for the purpose of food and only for shorter time like female mosquito, leech
  • Permanent parasites These parasites cannot live without their host and remain attach to host throughout their life like ascaris, lice etc.
  • Ectoparasites These parasites live on the body surface of host and suck blood, feed on living tissue or dead tissue of host like bird lice, aphids, fleas etc.
  • Endoparasites These parasites live inside the body of host and can be present intracellularly, inside tissues or body fluids. Some of the examples are Ascaris, Taenia, Trypanosoma etc.
  • Holoparasites and Hemiparasites Holoparasites are the parasites which are inseparable from their host and completely dependent on their host for everything like cuscuta, rafflesia Hemiparasites are partially dependent upon their host for nourishment like loranthus and viscum etc.


Amensalism is a symbiotic relationship between organisms of two different species in which one in which one organism is inhibited or destroyed while the other organism remains unaffected.  Organism releases a specific compound that harms another organism For example, Antibiotics,Bacteriocins, Antibacterial peptides (e.g., cecropins and defensins produce by insects and mammals, respectively), Acidic fermentation products. A classic example of amensalism is the production of antibiotics that can inhibit or kill the other microorganism.


This is a type of interaction in which one individual is benefitted while the other is neither harmed nor benefitted. Examples are sucker fish and pilot fish accompanies sharks for food, jackals follow lions or tigers for leftover food etc. Occurs in situations in which waste products of one microorganism serves as the substrate for another; also occurs in situations where one microorganism modifies the environment making it better suited for another microorganism some examples are given below

  • Nitrification-requires the activity of two different species; one oxidizes ammonia to nitrite and the other oxidizes nitrite to nitrate
  • The common nonpathogenic strain of Escherichia coli lives in the human colon; this facultative anaerobe uses oxygen creating an anaerobic environment in which obligate anaerobes (e.g., Bacteroides) can grow;
  • coli derives no obvious benefit or harm
  • Succession of microorganisms in an environment-during milk spoilage synthesis of acidic fermentation products by one population stimulates proliferation of acid-tolerant microorganisms; during biofilm formation, the first colonizer makes it possible for others to colonize
  • Colonization of surfaces of plants and animals by normal flora-plant or animal produces organic substances, which are used by the normal flora of the host organism


This is a type of interaction in which members of one species exploits another for its gain by capturing, killing and eating the prey. To catch the prey, predators have some adaptations like strength, catching and tearing organs etc. Predators also help in keeping check over preys and maintaining equilibrium, e.g., carnivorous animals eating up other animals, herbivorous animals eating up plants and carnivorous and insectivorous plants like Utricularia, Nepenthes etc.

Positive outcomes of predation

  • Microbial loop-microbial predators mineralize the organic matter produced by autotrophs (primary producers) before it reaches the higher consumers; this returns nutrients to the primary producers and promotes their activity
  • Ingestion of prey provides protective environment for the prey
  • Predatory fungi are known (e.g., fungi that trap nematodes)


 This is a type of mutualism interaction in which both individuals are mutually benefitted but can live without each other,