Membrane filtration is the method of choice for the analysis of fecal coliforms in water. Samples to be tested are passed through a filter of particular pore size (generally 0.45 micrometre). The microorganisms present in the water remain on the filter surface. The filter is placed in a sterile Petri dish with a selective medium, growth of the desired organisms is encouraged, while other non-target organisms is suppressed. Each cell develops into a separate colony, which can be counted directly, and the initial inoculum size can be determined. Typically sample volumes of 100 ml will be used for water testing and filtered, with the goal of achieving a final desirable colony density range of 20 to 60 colonies per filter. Contaminated sources may require dilution to achieve a "countable" membrane. The filter is placed on a Petri dish containing M-FC agar and incubated for 24 hours at 44.5 °C (112.1 degrees F). This elevated temperature heat shocks non-fecal bacteria and suppresses their growth. As the fecal coliform colonies grow they produce an acid (through fermenting lactose) that reacts with the aniline dye in the agar thus giving the colonies their blue color.