MICROBIOLOGY Study Guide - 2012

CHAPTER 7. ENVIRONMENTAL FACTORS AFFECTING MICROORGANISMS

7.3. BIOLOGICAL FACTORS

In their natural environment, microorganisms develop within complex communities known as microbiocenoses. Various Forms of interaction, or Symbiosis (from the Greek symbiosus — living together), are established between microorganisms and other living organisms. There are several types of symbiotic relationships: mutualism, commensalism, synergism, metabiosis, satellitism, antagonism, parasitism, etc.

Mutualism is the co-existence of organisms of different species that is mutually beneficial. Numerous cases of syntrophy are known in the microbial world, involving the mutual supply of essential nutrients or growth factors. For instance, lactic acid Bacteria accumulate lactic acid and establish a favorable environmental pH for Yeast proliferation. In turn, yeast Cells synthesize B-group Vitamins required for the growth of lactic acid bacteria.

Close mutualistic symbiosis is observed in the microorganisms of kefir grains, which harbor lactic acid bacteria, acetic acid bacteria, and Yeasts. Lichens serve as a classic example of mutualism among microorganisms. Within the lichen thallus, the fungal partner and the alga (or cyanobacterium) are so intimately associated that they form a unified vegetative body. Both partners benefit from this symbiosis: the fungus receives Organic compounds from the algal cells, while supplying the Algae with mineral salts and protecting them from desiccation.

Commensalism (from the Latin commensalis — sharing a table) is a type of co-existence in which only one partner benefits without causing any harm to the other. Commensal organisms include plant epiphytic microbes as well as the microorganisms inhabiting the gastrointestinal tract of humans and animals, which obtain necessary nutrients from the macroorganism.

Metabiosis is a form of interaction in which the metabolic products of one microbial species serve as nutritional substrates for another. For example, yeasts ferment sugars to produce carbon dioxide and ethanol, which are subsequently utilized by acetic acid bacteria that oxidize ethanol into acetic acid. Nitrifying bacteria oxidize ammonia produced by putrefactive bacteria during the ammonification of protein compounds. Metabiotic relationships form The basis of the biogeochemical cycling of matter in nature, driving the succession of different microbial groups.

Synergism is the enhancement of physiological Functions in different partners of a microbial association, resulting in an increased yield of end metabolic products.

Satellitism is a variant of metabiosis. This form of interaction is characterized by the satellite microbe releasing growth factors—such as vitamins and Amino Acids—into the environment, which stimulate the growth of the associated species. For instance, soil bacteria of the genus Azotobacter secrete vitamins that stimulate the proliferation of bacteria capable of converting organic phosphorus forms into inorganic ones.

Antagonism is a type of relationship where one microbial species suppresses the growth of another or causes its complete destruction. The phenomenon of microbial antagonism was first discovered by Pasteur and Joubert (1877), who observed the rapid death of Escherichia coli when co-cultured with Pseudomonas aeruginosa. Microbial antagonism is driven by several mechanisms:

✵ competition for nutrients, wherein different microbial species consume nutrients or oxygen from the environment at varying rates, allowing faster-growing species to proliferate more rapidly;

✵ The production of metabolic byproducts (such as organic acids and alcohols) by antagonist microbes that exert inhibitory effects on other species;

✵ the production of specific metabolic products—Antibiotics or bacteriocins—by certain microorganisms that have a lethal effect on other microbes.

Parasitism is a form of interaction in which parasitic microbes develop at the expense of nutrients from living cells of other organisms. Parasitic microbes include pathogenic bacteria that cause diseases in humans, animals, and plants. Parasitism is also observed between Bacteriophages and bacteria, mycoparasites and Fungi, and actinophages and actinomycetes. An example of parasitic interaction is the intracellular Replication of the small bacterium Bdellovibrio bacteriovorus within cells of bacteria belonging to the genus Pseudomonas.