Microbial Associations

natural or man-made communities of microorganisms, such as bacteria, yeasts, algae, and fungi.

Microbial associations develop through symbiosis or metabiosis. Certain species of microorganisms that constitute an association are not only usually resistant to the products of the vital activities of the other species in the association but in fact use those products as sources of energy, carbon, nitrogen, or growth factor.

Some microbial associations are evolutionarily ancient and very stable. One example is lichens, which consist of photosynthesizing algae and heterotrophic fungi. The mucilage canals of birch and oak are inhabited by yeasts that ferment sugar to ethyl alcohol. The alcohol is oxidized by acetic-acid bacteria to acetic acid, which can then be oxidized by fungi and bacteria to carbon dioxide and water. Microbial associations of anaerobes and aerobes are also formed in the soil. The aerobes use up the oxygen and thereby make possible the development of the anaerobes. Cellobiose and glucose, which form upon the destruction of plant remains by cellulose-decomposing bacteria, are metabolized by nitrogen-fixing bacteria; these, after decomposing, serve as a source of nitrogen nutrition for the cellulose bacteria. A common association is one of yeasts and lactic-acid bacteria, the yeasts being resistant to lactic acid and the bacteria to ethyl alcohol. The type includes the ferments used for kefir and rye dough.

A unique microbial association is the mucoid “tea fungus,” which consists of yeasts and acetic-acid bacteria, and is used in the home to produce a sour beverage. A stable, artificially created microbial association is the commercial “M” race of yeasts Saccharomyces cerevisiae, which consists of three different strains.