Topic 11: Special Media for Isolating Bacteria

• 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.
  • To help isolate organisms found in the minority, various enrichment and selective culturing methods are available that either enhance the growth of some organisms or inhibit the growth of other organisms.
• Selective media: contain 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.
  • usually liquid media
  • other bacteria will grow, but growth of the desired bacteria will be increased
• Differential media: contain various nutrients that allow the investigator to distinguish one bacterium from another by how they metabolize or change the media with a waste product.
  • dyes such as phenol red, eosin, or methylene blue are sometimes included
  • products of bacterial metabolism can react with the dyes to produce a color change in the medium
  • Eosin and methylene blue dyes in eosin methylene blue (EMB) agar is selective
  • inhibit the growth of some bacteria

\