Beneficial (favorable) relationships
These forms of symbiosis imply that microorganisms either help each other or, at the very least, do not interfere with each other's vital activity. There are four main variants of such relationships:
- Metabiosis
One species of microorganisms uses the metabolic products of another species for its vital activity, forming a unique metabolic chain. Example: Soil nitrifying bacteria use ammonia previously released by ammonifying bacteria.
- Mutualism
A classic mutually beneficial cohabitation in which both partners win. Example: Lichens, which represent an inseparable symbiosis of a fungus and a blue-green alga. In this association, there is a clear division of functions: the fungus receives essential organic substances from the alga, while the alga takes mineral salts from the fungus and receives reliable protection against drying out.
- Commensalism
The term originates from the word commensalis ("table companion"). This is a form of interaction in which one species derives a benefit for itself while causing no harm whatsoever to its neighbor. Example: Representatives of the normal human microflora (commensal bacteria) living on the skin and mucous membranes.
- Satellitism
A phenomenon in which the growth of one species is enhanced under the favorable influence of another. The mechanism involves the secretion of specific metabolites that stimulate the development of neighboring microbial colonies. Joint growth activates physiological functions and significantly accelerates the processing of the nutrient substrate. Example: Yeast or Sarcina colonies actively stimulate the growth of other microorganisms surrounding them.
Antagonistic symbiosis
Antagonistic relationships are the detrimental effect of one microbial species on another, which inevitably leads to the suppression of the competitor's growth or its complete death. Such relationships are widely prevalent in soil, water, as well as in the bodies of animals and humans.
Colonization resistance is of immense clinical significance. This is a defense mechanism in which the normal microflora (e.g., bifidobacteria, lactobacilli, and Escherichia coli in the large intestine) exhibits pronounced antagonism toward foreign and pathogenic putrefactive flora, preventing them from colonizing the mucous membranes.
Main mechanisms of suppression (advantage factors):
- High reproduction rate. Allows the microbe to rapidly seize a free ecological niche and deprive competitors of resources.
- Antibiotic production. These are specific metabolic products that purposefully suppress the development of competitors. This is the most common form of microbial antagonism.
- Bacteriocin production. Synthesis of specific antimicrobial proteinaceous substances (specifically, colicins).
- Alteration of the physicochemical properties of the environment. The production of organic acids by microbes leads to a sharp decrease in the pH of the medium. Acidification makes conditions unsuitable for the survival of competing species.
Ecological forms of antagonism
Microbial antagonism is realized in nature through several rigid ecological scenarios aimed at the survival of the fittest:
- Competition. Direct struggle for limited food sources. A more aggressive or fast-growing antagonist depletes the nutrient medium, thereby suppressing the growth and reproduction of other species.
- Predation. Physical capture and digestion of enemy cells. A classic example from the microworld is the intestinal amoeba, which actively engulfs and digests bacteria.
- Parasitism. Using another organism as a source of nutrition and habitat, which ultimately leads to the death of the host. Vivid examples: bacteriophages (bacterial viruses that destroy bacterial cells) and Bdellovibrio (small parasitic bacteria that attack gram-negative bacteria).
Briefly: answers to topical questions
What are the differences between commensalism and mutualism?
| Form of symbiosis | Characteristic |
|---|---|
| Mutualism | Both symbionts receive mutual benefit; mutually beneficial symbiosis |
| Commensalism | Partnership is beneficial to one without harmful effects on the other |
In mutualism, there is a mutually beneficial cohabitation of different organisms where each participant benefits. For example, in lichens, the fungus receives organic substances from the alga, while the alga receives mineral salts and protection from drying out. In commensalism, one species benefits without harming the other. The microbe lives at the expense of the host, uses its protection, but causes no harm. Representatives of the normal human microflora inhabit the skin and mucous membranes as commensals.
What types of bacteriocins are synthesized by microorganisms?
Microorganisms synthesize specific antimicrobial substances—bacteriocins, specifically colicins. Bacteriocins are encoded by special Col-plasmids of bacteriocinogeny. These substances kill bacteria of other strains or closely related species. For example, the Col-plasmid in E. coli encodes colicins acting on coliform bacteria, which provides the carrier bacteria with a competitive advantage when colonizing ecological niches.
To memorize the forms of beneficial symbiosis, use the abbreviation MMCS: Metabiosis, Mutualism, Commensalism, Satellitism.
Class and exam questions
What is colonization resistance?
This is the ability of the body's normal microflora (e.g., intestinal lacto- and bifidobacteria) to suppress the colonization and reproduction of foreign pathogenic or putrefactive flora.
What is the essence of satellitism?
In satellitism, one microorganism secretes metabolites that act as growth stimulators for colonies of neighboring species, accelerating the joint processing of the substrate.
How do microbes alter their habitat to suppress competitors?
They can produce organic acids. This leads to a sharp change (decrease) in the pH of the medium, making it unsuitable for the life of competing species.
What else to study on this topic
- Mechanisms of action of bacteriocins and colicins
- Properties and types of antibiotics of microbial origin
- The role of the normal microflora of the large intestine
- Bacteriophages: life cycle of bacterial viruses
- Bdellovibrio: unique parasitic bacteria
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