The dyes eosin and methylene blue inhibit the growth of gram positive organisms and select for gram negative organisms. EMB agar also contains lactose. Coliforms ferment the lactose, which causes the darkening of the colonies. Some coliforms have different appearances on EMB agar. On EMB agar, non-lactose fermenters produce pale or colorless colonies.
Many intestinal pathogens are non-lactose fermenters.
The United States Environmental Protection Agency has established guidelines for evaluating the bacteriologic quality of water for drinking. The purpose of this exercise is to determine whether or not a water sample is contaminated with coliforms using the presumptive and confirmed stages of testing. The standard method lists gas production in lactose broth as a positive presumptive test for coliforms. Collection might be best assigned over a weekend to allow time for investigation of potential sites. Use the plastic dropper pipet to inoculate the tube of lactose broth with 1ml of the water sample.