Special Media for Isolating Bacteria Because multiple methods and multiple med exist, you must be able to match the correct procedu to the desired mierobe. For example, if bacterium B salt-tolerant, a high concentration (>5%) of salt cou be added to the culture medium. Physical conditio can also be used to select for a bacterium. If bacte OBJECTIVES After completing this exercise, you should be able to: 1. Differentiate selective from differential media. 2. Provide an application for enrichment and selec- tive media. um B is heat-resistant, the specimen could be heat before isolation. Dyes such as phenol red, eosin, methylene blue are sometimes included in differenti media. Products of bacterial metabolism can react wi BACKGROUND One of the major limitations of dilution techniques used to isolate bacteria is that organisms present in limited amounts may be diluted out on plates filled with dominant bacteria. For example, if the culture to be isolated has 1 million of bacterium A and only 1 of bacterium B, bacterium B will probably be limited to the first sector in a streak plate. To help isolate organ- isms found in the minority, various enrichment and selective culturing methods are available that either MATERIALS enhance the growth of some organisms or inhibit the growth of other organisms. Selective media con- tain chemicals that prevent the growth of unwanted bacteria without inhibiting the growth of the desired organism. Enrichment media contain chemicals that enhance the growth of desired bacteria. Other bacteria will grow, but the growth of the desired bacteria will be these dyes to produce a color change in the medium colonies (FIGURE1 and FIGURE 2). The dyes (eosin ar methylene blue) in eosin methylene blue (EMB) ag are also selective. These dyes inhibit the growth some bacteria. Three culture media will be compar in this exercise (TABLE 1). FIRST PERIOD Petri plates containing nutrient agar (2) Petri plates containing mannitol salt agar (2) Petri plates containing EMB agar (2) SECOND PERIOD Gram-staining reagents BSL-1 Demonstration plate (sealed) with Staphylococcus aureus on one half and Staphylococcus epidermidis on the other half. increased. Another category of media useful in identifying bacteria is differential media. These media contain various nutrients that allow the investigator to distin- guish one bacterium from another by how they metab- olize or change the media with a waste product. SBOCEDNHE U TABLE 1 MAJOR CHEMICAL COMPONENTS OF MEDIA USED IN THIS EXERCISE Chemical Components Nutrient Agar Mannitol Salt Agar EMB Agar Peptone 0.5% 1.0% 1.0%