An indicator in the 3M™ Petrifilm™ Enterobacteriaceae Count Plate colors all colonies red. The top film traps gas produced by some bacteria. Acid-producing bacteria are seen as red colonies surrounded by yellow zones. Bacteria producing gas and/or acid are considered to be presumptive Enterobacteriaceae and will have one of the following characteristics on the 3M Petrifilm Enterobacteriaceae Count plate: colonies associated with gas bubbles and no acid zones, colonies with yellow acid zones but no gas production, or colonies producing both gas and acid.