Coliform colonies growing on the Petrifilm CC plate produce acid which causes the pH indicator to deepen the gel color.
Gas trapped around red colonies indicates coliforms.
ISO defines coliforms by their ability to grow in method-specific, selective media. Notice the change in gel color in figures 1 through 4. As the coliform count increases, the color of the gel deepens from pink to a deep red. The identification of coliforms vary by country. Artifact bubbles may result from improper inoculation of the Petrifilm Coliform Count plate.