GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

21. MICROORGANISMS AND THE ENVIRONMENT

21.3. TYPES OF RELATIONSHIPS BETWEEN ORGANISMS IN NATURE

21.3.1. Symbiotic relationships

Symbiotic, or associative, relationships are characterized by mutual benefit. They are considered positive interactions between organisms, even if only one partner benefits from living together with the other.

Symbiosis is a form of coexistence in which both symbionts benefit from living together. The classical form of symbiosis represents a coexistence where, under certain conditions, the symbionts cannot exist separately from one another. An example is the cohabitation of an ascomycete fungus and a specific species of Algae, whose associations form a new Organism — Lichens. This association achieves mutual benefit only under very specific ecological conditions — namely, under nutrient deficiency and drought. The fungus supplies the alga with minerals and Water from the environment and protects algal Cells from adverse factors. The alga, as an organism capable of assimilating CO₂, provides the fungus with Photosynthesis products and the products of MOLECULAR Nitrogen Fixation from the air. Other Examples of fungal symbiosis with various organisms were also discussed in the chapter "Fungi".

In the symbiosis between rhizobial Bacteria and legumes, both associates benefit from the cohabitation, although the ties between them are less rigid than in organisms forming lichens. A variation of this symbiosis is called mutualism or cooperation. Both the bacteria and the host plant can grow and develop independently. During coexistence, rhizobial bacteria fix nitrogen, and the products of nitrogen fixation are transferred to the plant, providing it with available nitrogen compounds, while the bacteria receive the products of CO₂ assimilation produced within the plant.

There is a form of symbiosis between microorganisms and macroorganisms called commensalism, in which symbionts benefit from protection or nutrients provided by the macroorganism without benefiting it, yet without causing it any harm. An example is the relationship between The Human Body and its normal microflora, such as the saprophytic microflora of the Upper Respiratory Tract, Skin, and digestive tract. Members of the normal microflora inhibit the growth of pathogenic microorganisms, synthesize growth factors, and break down complex CARBOHYDRATES.

Symbiotic relationships within microbial communities can also manifest as syntrophic. Syntrophy is The phenomenon of joint growth of two or more microbial species on a medium inaccessible to each species individually. The most common type of syntrophic interaction is the exchange of growth factors or substrates. For instance, symbiosis in lactic acid bacteria occurs because each symbiont synthesizes and releases into the medium substances required by the other partners.

When stimulation results from a unilateral effect, this type of relationship is called satellitism. For example, during co-culture on Agar medium, microorganisms that do not produce Vitamin B12 proliferate around colonies of microorganisms that synthesize cyanocobalamin.

Syntrophic interaction can also manifest when one microbial species prepares the ground for The Development of another.

This type of syntrophy is called metabiosis. Metabiosis is a form of interaction in which the metabolic products of one microorganism serve as a substrate (nutritional or energetic) for another. For example, nitrifying bacteria of the genus Nitrosomonas, by oxidizing ammonia to nitrites, provide an energy substrate for bacteria of the genus Nitrobacter.

Synergism is a form of microbial coexistence in which the associates mutually enhance each other's physiological Functions and acquire new properties. An example is the cohabitation of acetic acid bacteria and Yeast (the "kombucha" culture). Acetic acid bacteria convert sucrose into glucose and fructose, and oxidize Monosaccharides into gluconic and oxoglutaric acids, which are utilized by the yeast. The yeast, in turn, synthesizes Vitamins and supplies them to the acetic acid bacteria.