Microbial Interactions in the Environment


  • In microbial ecology, microorganisms are associated with higher organisms.
  • The term symbiosis (“together- life”), elaborates various interactions between microorganisms and also between higher organisms such as plants and animals. It may be positive or negative interactions.
  • The interaction includes as follows:
    • Mutualism
    • Commensalism
    • Parasitism
    • Predation
    • Amenalism
    • Competition
  • Its occurrence of these interactions is natural, it may cause disease.
  • In various ways, microorganisms forms association with other higher organisms.
    • Some are ectosymbiont; one organism can be found on the surface of the other organisms.
    • Some are endosymbiont; one organism can be found within the other organisms.
    • Usually, simple microbial interactions include a symbiont and a host.
    • There is a number of hosts that have more than one symbiont, known as a consortium. For e.g., Thiothrix spp, a sulfur bacteria, located on the mayfly larva’s outer surface which contains a parasitic bacterium.
    • These physical associations are either transient or permanent.
    • In transient associations, pathogen-causing human diseases which include listeriosis, malaria, and leptospirosis are involved.
    • In permanent associations, both the host and the symbiont are coevolved, and the symbiont benefits the host.


  • Mutualism (derived from Latin mutuus which means borrowed or reciprocal), in this relationship both partners are benefitted.
  • It is considered as an obligatory relationship, in which both symbiont and host are dependent on each other.
  • In many cases, individual organisms can’t survive in absence of the symbiont.
  • Several e.g., of mutualism, are as follows:

Microorganism-Insect Mutualisms

  • The mutualistic interactions are seen often in the insects because food intake such as plant sap or animal fluids lack in necessary vitamins and amino acids consumed by the insects.
  • Therefore bacterial symbiont provides essential nutrients, in exchange for required nutrients and habitat.
  • For instance, aphids inhabit proteobacteria Buchnera aphidicola in the cell cytoplasm, it provides 10 essential amino acids.
  • B. aphidicola is considered as an obligate mutualistic symbiont, both host and symbiont are coevolved and can’t survive in absence of either partner.
  • One more example is the protozoan-termite relationship; flagellated protozoan mutually resides in the guts of the wood roaches and termites.
  • The host ingests cellulose which is digested by the enzyme cellulase produce by the protozoa which is metabolizing into acetate and other products.
  • Experimentally there coexistence is determined, termites placed in a jar with a high concentration of O2 and wood chips, hence oxygen is toxic to the flagellates, later it is observed termites die because of starvation, due to their inability to digest cellulose which is carried out by the flagellates.


  • Most of the marine invertebrates like sponges, jellyfish, corals, and ciliates inhabit endosymbiotic dinoflagellates within the tissues known as zooxanthellae.
  • The reef-building corals obtain their energy requirements with the help of zooxanthellae which are mutually associated with them.
  • 95% of carbon is fixated by these endosymbionts in return they receive phosphates, CO2, nitrogenous base, and UV protection.
  • Due to global warming, there are increased cases of coral bleaching, which means deprivation of photosynthetic pigments from the corals or removal of endosymbiont i.e., zooxanthellae.
  • Due to the increased temperature of the oceans, and increased stressors, the photosystem II of the endosymbiont zooxanthellae is damaged and produces reactive oxygen species (ROS), which is toxic for them. Eventually causing coral bleaching.

Methane-based mutualism

  • Methanotrophs are capable of oxidizing methane and are found as an intracellular symbiont in the gills of the mussels present in the methane vent.
  • Even from the mud volcano of Barbados Trench, methanotrophic carnivorous sponges have been obtained.
  • This methanotrophic endosymbiont decreases the flux of methane in the peat bogs (wetland that accumulates dead plant material, especially Sphagnum mosses).
  • These mosses contain proteobacteria residing within the outer cortex cells of the stem which help to oxidize methane.

Rumen-microbe mutualism

  • This is an excellent example of mutualism which is well studied.
  • Ruminants include cattle, deer, buffalo camels, sheep, goats, etc, they evolved an “eat now, digest later” technique, in which they chew their cud i.e., partially digested cellulose.
  • Their stomach is divided into four chambers: The upper part is Rumen, the Lower portion is followed by antechamber, the omasum, and abomasum (true stomach).
  • Rumen contains a diversity of microorganisms which include bacteria, archaea, protists and, fungi, which carry out anaerobic fermentation.
  • One bacterial population, present in the rumen secrete extracellular cellulases which cleave the glycosidic bond beta  (1- 4) present in the cellulose.
  • Fermentation product by bacteria in the rumen is acetate, butyric acid, and propionate, animal obtains organic acid and fatty acid by symbiotic bacteria which serves as a sole source of the energy.
  • In some ruminants like cows, acetate, carbon dioxide, and H2 obtained by anaerobic fermentation are used by methanogenic archaea to produce methane i.e., a greenhouse gas.
  • Methanogens also provide essential vitamins required by ruminants.


  • Cooperation is considered a positive association, which involves syntrophic relationships.
  • The term “syntrophism” (Greek syn, together and trophe, nourishment), is a relationship in which one organism’s growth relies upon or is improved by nutrients obtained from another organism growing in proximity.
  • In some cases, both organisms are benefitted.
  • The most useful difference between cooperation and mutualism is, in cooperation, both organisms can be separated from one another and still survive, but it may hamper their functions.
  • The two examples of a cooperative association, are as follows:
    • Desulfovibrio and Chromatium, which participate in the sulfur cycle and carbon cycle respectively. Both cycles are linked because the organic matter & sulfate needed by Desulfovibrio are produced by the Chromatium, which goes through photosynthesis-driven reduction of carbon dioxide and organic matter and oxidizes sulfide to sulfate.
Cooperative Association between Desulfovibrio and Chromatium
Cooperative Association between Desulfovibrio and Chromatium
    • One more example, the nitrogen-fixing bacteria interacts with a cellulolytic organism like Cellulomonas; it degrades cellulose and liberates glucose which is further used by nitrogen-fixing bacteria.
Cooperative association between Cellulomonas and Azotobacter
Cooperative association between Cellulomonas and Azotobacter
  • One example of cooperative biodegradation association, for degradation of toxin 3- cholorobenzoate , complementary group of microorganisms work together, if any participate is absent, degradation of the substrate will not be carried out.


  • Commensalism (Latin origin, “com” means together, and “mensa” means table), is an association in which individual symbiont, i.e., the commensal, benefitted whereas the host is neither benefitted nor harmed, it’s a unidirectional process.
  • The commensal feeds on nutrients obtained from the host, it occupies shelter within or on the host.
  • Commensal is not directly involved with the host’s metabolism and cause no harm to the host and can live without the host.
  • Even commensalism is also seen among the microorganisms community where waste product generated by the microorganism is a substrate for other species. For e.g., in the nitrification process, Nitromonas oxidize ammonium ion to nitrite, then subsequently oxidation of nitrite to nitrate is carried out by Nitrobacter.
  • Another e.g. is in anoxic methanogenic ecosystems like sludge digesters, flooded soils; two different bacterial groups interact and degrade fatty acid to produce H2 and methane. Methanogens produce methane, which is carried out by interspecies hydrogen transfer, and hydrogen gas is obtained by fermentative bacteria which is quickly used by methanogens
  • One more idea of commensalism relationship is when one microorganism modifies the surroundings to make it more favorable for other microorganisms.
    • For e.g., nonpathogenic strains of E.coli found in the human colon, can grow well outside the host and are typical commensal. The facultative anaerobes E.coli used all present oxygen which makes obligate anaerobes Bacteroides to grow well within the colon.
  • Even commensalism, involves other environmental alterations, like a synthesis of more acidic waste products promote the growth of the more caid tolerant bacteria. Another example is biofilm formation.
  • In the human body, there are various microbiotas present which is commensal, for e.g., the skin produces volatile, soluble, organic compounds which serve as a nutrient for microorganisms present on the skin.


  • Among microbes predation involves, predator species attacking and usually kills their prey.
  • There are several predator species of the microbes which include Bdellovibrio, Vampirecoccus, and Daptobacter.
  • Bdellovibrio is motile, and susceptible prey is gram-negative bacteria. They pierce a hole in the outer membrane of the prey and enter the periplasmic space, progeny bacterial cells of Bdellovibrio are produced when the prey cell is lysed.
  • Vampirococcus, attach itself to the exterior membrane of the prey as an epibiont and secretes enzymes that degrade the outer membrane which results in oozing out of the cytoplasmic content of the prey.
  • Daptobacter directly feeds on the cytoplasmic content of the prey by penetrating inside the cell.
  • Predation is also advantageous:
    • It increases the rates of the cycle of nutrients.
    • It’s important for the proper functioning of the microbial loop, in this process, organic matter is mineralized through photosynthetic and chemosynthetic activity before it is consumed by higher organisms.
    • Even predation provides a protective and high nutrient environment for the particular prey. For e.g., Ciliates ingest gram +ve bacterium Legionella and protect the pathogen from chlorine which is toxic. It is also observed to increase their pathogenicity.
    • Fungi also have predatory characteristics; few fungi can trap protozoa with the help of the sticky hyphae or knobs, or sticky meshwork of hyphae. For e.g., Arthrobotyrs traps the nematode, with the help of the constricting rings, then hyphae grow over the immobilized prey and their cytoplasmic contents are used as nutrients.


  • It is considered as the most complex microbial interaction, in this association between two organisms, one is benefitted and the other is harmed.
  • It includes the acquisition of nutrients of the host, co-existence is observed in the case of parasitism between parasite and host.
  • It is stated that successful parasitism, is considered when parasites coexist with the host in an equilibrium.
  • Because, if the host dies immediately after the invasion, it will prevent the reproduction of the parasitic microbes due to that they won’t be able to colonize in the new host.
  • Parasitism fails when the host has a strong immune response or went through antimicrobial therapy.
  • If the host-parasitic relationship is established, it favors the growth of the parasite inside the host, for e.g., a disease caused by the Rickettsia typhi.
  • The controlled parasitic-host relationship is also observed in nature and can maintain their association for a longer period of time.
  • E.g., Lichens, which are the fungal and algal association, previously it was kept under mutualism category, but research has been carried out to state that lichen is formed when both potential partners are nutritionally deprived.
    • The algal part is known as phycobiont and the fungal part is known as the mycobiont. As we know algae are photoautotrophic in nature, they rely on light, CO2, and mineral nutrients.
    • Whereas fungi obtain carbon directly from the alga, and nutrients are obtained by fungi through their extended fungal projection called haustoria which penetrates the algal cell wall, because of this invasive nature it is categorized into parasitism.
    • Fungal protect alga from high light intensities, also provides water and mineral, and for the protected growth of phycobiont from environmental stress, it creates a firm substratum.


  • Amensalism state the harmful effects that one organism has on other organisms.
  • Basically, it is a unidirectional process; it involves one organism producing certain compounds which have a negative impact on the other organism.
  • For e.g., Antibiotic production by one microorganism can kill or inhibits a susceptible range of microorganisms.
  • Other e.g., are some microorganisms that produce organic compounds such as bacteriocins that cause cell wall disruption of the target microorganism


  • Competition arises when there is a limiting nutrient source, and at the same time, different organisms within a population are trying to obtain that same resource.
  • If one organism dominates the environment by occupying the physical space or acquiring the nutrient, it’ll outgrow another participant.
  • This phenomenon was purposed by E.F.Gauser in 1934, as a competitive exclusion principle.
  • In this case, both organisms cannot coexist, but at a low population, they can also coexist.
  • E.g., in a chemostat, there is a limiting nutrient resource, slow-growing microorganisms are excluded, changes in dilution can make the slow-growing bacteria dominant.

Overview of Microbial Interactions at Glance

Microbial Interactions
Microbial Interactions

Reference and Sources

  • https://fyupmicrobiology.blogspot.com/
  • https://dokumen.pub/microbiome-in-plant-health-and-disease-challenges-and-opportunities-1sted-
  • https://www.slideshare.net/NOMIKhanS/interaction-of-microbes-with-eukaryotes
  • https://www.scribd.com/presentation/362979036/interaksi-mikrobia
  • https://www.biologydiscussion.com/microbiology-2/microbial-associations/symbiotic-relationship-betweenorganisms-
  • https://www.slideshare.net/yusuralani/interactions-between-microorganisms-and-plants
  • https://www.deepdyve.com/lp/springer-journals/the-importance-of-methane-and-thiosulfate-in-themetabolism-
  • https://physicsgurukul.com/2021/05/17/assertion-reason-questions-for-biology-chapter-2-biologicalclassification/
  • https://www.researchgate.net/publication/334140734_Genome_Evolution_of_the_Obligate_Endosymbiont_Bu
  • https://www.sciencedirect.com/topics/immunology-and-microbiology/buchnera-aphidicola
  • https://www.academia.edu/5626826/Ecology_and_Symbiosis_of_microbiology

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