What color are the fecal colonies on the membrane filtration method of bacterial analysis?

In the membrane filtration method of bacterial analysis, the fecal colonies typically appear blue. This coloration is a result of specific indicators used in the agar medium. For instance, certain selective media are designed to highlight coliform bacteria, which are often associated with fecal contamination. By incorporating an indicator such as X-gal or similar, which produces a blue color when metabolized by bacteria, the presence of fecal coliforms can be easily identified.

The choice of color is significant because it provides a visual cue for microbiologists assessing water quality and safety. Accurate identification of these colonies is essential for determining contamination levels and ensuring compliance with health standards. Understanding this visual identification method is crucial for wastewater treatment operators and public health officials monitoring water supply safety.