Microbiology

Microbiology Study Guide

Cultivation Media When microorganisms are cultivated in the laboratory, a growth environment called a medium is used. The medium may be purely chemical (a chemically defined medium), it may contain organic materials, or it may consist of living organisms, such as fertilized eggs. Microorganisms growing in or on such a medium form a culture. A culture is considered a pure culture if only one type of organism is present and a mixed culture if populations of different organisms are present. When first used, the culture medium should be sterile, meaning that no form of life is present before inoculation with the microorganism. General microbial media: For the cultivation of bacteria, a commonly used medium is nutrient broth, a liquid containing proteins, salts, and growth enhancers that will support many bacteria. To solidify the medium, an agent such as agar is added. Agar is a polysaccharide that adds no nutrients to a medium, but merely solidifies it. The medium that results is nutrient agar. For anaerobic microorganisms, the atmosphere must be free of oxygen. To eliminate the oxygen, the culture media can be placed within containers where carbon dioxide and hydrogen gas are generated and oxygen is removed from the atmosphere. Commercially available products achieve these conditions. Anaerobic chambers can also be used within closed compartments, and technicians can manipulate culture media within these chambers. To encourage carbon dioxide formation, a candle can be burned to use up oxygen and replace it with carbon dioxide. Special microbial media: Certain microorganisms are cultivated in selective media . These media retard the growth of unwanted organisms while encouraging the growth of the organisms desired. For example, mannitol salt agar is selective for staphylococci because most other bacteria cannot grow in its high-salt environment. Another selective medium is brilliant green agar, a medium that inhibits gram-positive bacteria while permitting gram-negative organisms, such as Salmonella species, to grow. Still other culture media are differential media. These media provide environments in which different bacteria can be distinguished fromone another. For instance, violet red bile agar is used to distinguish coliform bacteria, such as Escherichia coli, fromnon-coliformorganisms. The coliform bacteria appear as bright pink colonies in this media, while non-coliforms appear a light pink or clear. Certain media are both selective and differential. For instance, MacConkey agar differentiates lactose- fermenting bacteria from non-lactose-fermenting bacteria, while inhibiting the growth of gram- positive bacteria. Since lactose-fermenting bacteria are often involved in water pollution, they can be distinguished by adding samples of water to MacConkey agar and waiting for growth to appear. In some cases, it is necessary to formulate an enriched medium. Such a medium provides specific nutrients that encourage selected species of microorganisms to flourish in a mixed sample. When attempting to isolate Salmonella species from fecal samples, for instance, it is helpful to place a sample of the material in an enriched medium to encourage Salmonella species to multiply before the isolation techniques begin.

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