In the United States, coliform bacteria serve as the indicator organisms for fecal contamination. Coliforms are gram negative, aerobic or facultative anaerobic rods that are consistently present in human feces in substantial numbers. Microbiologists investigating the microbiota of an environment generally find a mixture of organisms. In order to study one organism, it must first be separated from the mixture or isolated . Microbiologists use the steak plate technique as well as special media to isolate desired organisms.
Some media also contain dyes or chemicals that select for a desired organism.
These are called selective media . Other media, called differential media , may have chemicals added that will demonstrate a metabolic property of certain microorganisms. Some media are both selective and differential. The dyes eosin and methylene blue inhibit the growth of gram positive organisms and select for gram negative organisms. EMB agar also contains lactose.