Enumeration Of E. Coli And Coliforms In Food Products And Food Ingredients Using 3M^TM Petrifilm^TM E. coli Count Plates

HPB Method MFHPB-34
February 2001

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1. APPLICATION

This method is applicable for the enumeration of Escherichia coli and other coliforms in food products and food ingredients. This product may also be used for environmental sampling (see Laboratory Procedure MFLP-41A and MFLP-41B). This revised method replaces MFHPB-34, dated April 1997.

Do not use this plate alone for the detection of E. coli O157. Like most other E. coli and coliform media, this plate will not specifically indicate whether any O157 strain is present. Because most O157 strains are atypical E. coli, they will appear as non-E. coli coliforms (red with gas).

2. PRINCIPLE

Petrifilm plates are a ready-to-use product developed by the 3M Company, St. Paul, MN. Media are coated onto films, so that traditional media preparation is unnecessary, and both labour and time savings are realized. E. coli and other coliforms can be enumerated on Petrifilm E. coli Count (EC) plates. The method uses bacterial culture plates containing dry medium and a cold-water-soluble gelling agent. One mL samples are added directly to the plates. Pressure, when applied to the plastic spreader placed on the overlay film, spreads the sample over 20 cm². The gelling agent is allowed to solidify, and the plates are then incubated and counted. Validation and collaborative studies have found the Petrifilm EC method to be not significantly different from the traditional methods (7.1, 7.2, 7.5 - 7.8, 7.10, 7.11, 7.14, 7.15).

3. DEFINITION OF TERMS

3.1 See Appendix A of Volume 2.

3.2 Petrifilm EC plates contain modified Violet Red Bile (VRB) nutrients; a cold-water-soluble gelling agent; 5-bromo-4-chloro-3-indolyl-ß-D-glucuronide (BCIG), an indicator of glucuronidase activity; and 2, 3, 5-triphenyl tetrazolium chloride (TTC) indicator. Most E. coli contain a glucuronidase enzyme (7.15). When glucuronidase is present, the BCIG indicator is cleaved and blue colonies are formed, differentiating E. coli from other coliforms.