Enrichment Media. Natural environments are usually populated by numerous kinds of bacteria or other microorganisms. When a species of special interest is present,but only in very small numbers,microbiologists use an enrichment medium. The medium favors the growth of that species,but not the growth of the others present in the mixed population. Enrichment techiques provide an environment, both chemical and physical, that results in increased numbers of an initially scarce species. Unlike a selective medium, no inhibitory agent is used to prevent the growth of unwanted microorganisms. After serial transfers of growth to new media preparations,the desired species emerges as the predominant enriched population. For example, phenol-oxidizing bacteria can be isolated from soil samples by using an ammoniumsalts medium with phenol as the only source of carbon and energy. Only microbes able to oxidize phenol will be present in any large numbers after several serial transfers