Background:  Coliform bacteria are a group of bacteria which are commonly used as a marker or index of the potential presence of pathogens (i.e. food safety) and as indicators of overall food quality and the hygienic conditions present during food processing. Coliforms are ubiquitous in nature, therefore a number of factors should be considered when testing for a particular indicator organism such as the native microflora of the food, the extent to which the food has been processed, and the effect that processing would be expected to have on the indicator organisms. This testing method employed to detect coliforms in a sample is simultaneously used to detect E. coli as well.

Analysis Description:  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 Petrifilm EC plate (within approximately one colony diameter).

AOAC International and U.S. FDA Bacteriological Analytical Manual (BAM) define coliforms as gram-negative rods which produce acid and gas from lactose during metabolic fermentation.  Coliform colonies growing on the 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.

The sample is diluted with a buffer, blended, and an extracted amount is plated on the Petrifilm, many times in a serial dilution format.  Inoculated Petrifilm are incubated at 25°C for 24 to 48 hours.