Final answer:
Selective media like MacConkey agar select for gram-negative bacteria by inhibiting gram-positive ones. Enrichment cultures support specific organisms, such as hydrocarbonoclastic bacteria. Differential media distinguish bacteria based on colony color, aiding identification of pathogens such as EHEC and other E. coli strains.
Step-by-step explanation:
The organisms being selected for and against in selective media are determined by the ingredients of the media that favor the growth of certain bacteria while inhibiting others. An example of a selective medium is MacConkey agar, which selects for gram-negative bacteria, such as those from the Enterobacteriaceae family, by inhibiting the growth of gram-positive bacteria through the presence of bile salts and crystal violet.
Enrichment cultures are designed to foster the growth of particular microorganisms. For instance, to enrich cultures with oil-eating bacteria, a medium would provide carbon only in the form of crude oil. Differential media are used to distinguish different bacteria, often by a color change in the colonies or the medium itself which results from bacterial enzymes interacting with substrates in the medium. In the case of hemolytic reactions, this can involve the lysis of red blood cells.
Using selective and differential media, bacteria such as EHEC (enterohemorrhagic E. coli), other strains of E. coli, S. aureus, B. cereus, Salmonella, and Shigella can be isolated and identified from food samples. Observations such as colony color changes on the medium help determine the presence of these types of bacteria.