BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.2. Culture media

12.2.2. Enriched and selective media

An enriched medium is designed with components tailored to meet the specific nutritional needs of particular microorganisms, thereby giving them a growth advantage over others. When a mixed culture, typically sourced from a natural environment, is introduced into such a medium, one or a few types of microorganisms will generally outgrow the rest. By using the appropriate medium, a desired microorganism can be selectively cultivated. Adjusting parameters such as Temperature, pH, carbon sources, and Energy Sources allows researchers to find the optimal combination for a given Organism. For example, Salmonella Bacteria, which cause typhoid fever (Salmonella typhi), are isolated and identified by placing food or fecal samples into a medium containing selenium derivatives (selenium being an element chemically similar to sulfur). Common enteric bacteria such as E. coli, which may also be present in feces, are inhibited by selenium. Enrichment culture stands as one of the most powerful tools available to microbiologists for isolating microorganisms.

A selective medium is one supplemented with substances that inhibit the growth of all organisms except one or a few specific types. Examples include adding penicillin to a culture to select for resistant strains, or selecting hybrid Cells during The production of Monoclonal Antibodies (Section 12.11.2).

12.4. How would you isolate an organism from soil that utilizes atmospheric nitrogen as its sole nitrogen source (a nitrogen-fixing organism)?



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