3M™ Petrifilm™ E. coli/Coliform Count (EC) Plates contain Violet Red Bile (VRB) nutrients, a cold-water-soluble gelling agent, an indicator of glucuronidase activity, and an indicator that facilitates colony enumeration. Most E. coli (about 97%) produce beta-glucuronidase which produces a blue precipitate associated with the colony. The top film traps gas produced by the lactose fermenting coliforms and E. coli. About 95% of E. coli produce gas, indicated by blue to red-blue colonies associated with entrapped gas on the 3M Petrifilm EC Plate (within approximately one colony diameter).
U.S. Food and Drug Administration Bacteriological Analytical Manual (FDA-BAM) define coliforms as Gram-negative rods which produce acid and gas from lactose during metabolic fermentation. Coliform colonies growing on the 3M Petrifilm EC Plate produce acid which causes the pH indicator to make the gel color darker red. Gas trapped around red coliform colonies indicates confirmed coliforms.
*Do not use this plate alone for the detection of E. coli O157. Like most other E. coli/Coliform media, this plate will not specifically indicate whether any O157 strain is present.