Petrifilm E. Coli

Interpretation Guide

Petrifilm™
E. coli and Coliform Count Plates
This guide should familiarize you with results on Petrifilm™ E. coli and Coliform count plate (EC). For more information contact the 3M Microbiology Products representative in your area. Petrifilm EC plates contain VRB nutrients, a cold-water-soluble gelling agent, an indicator of glucuronidase activity BCIG, and a tetrazoliumindicator that facilitates colony enumeration. The top film traps gas produced by the lactose fermenting Coliforms and E. coli. • Time and Temperature of incubation as well as interpretation of the Petrifilm EC plates vary by method. Therefore, this may give slightly different results. Temperatures of incubation used in the most common methods are mentioned in the package insert, and examples areshown in the technical section (pages 2 and 3) of this Guide. • Do not count colonies on the foam dam since they are removed from the selective influence of the medium. • Follow the time and temperature usually used in the laboratory. These are the most common used temperatures (E. coli and coliform) : 35, 37, 42 or 44°C during 24 to 48h.

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E. coli :
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E. coli are able to grow on mediacontaining Violet Red Bile (VRB) nutrients. Most E. coli (about 97%) produce beta-glucuronidase which reacts with a BCIG indicator dye in the Petrifilm EC plate that makes the colony turn blue to red-blue. About 95% of E. coli produce gas from lactose, this is indicated by colonies associated (within approximately one colony diameter) with entrapped gas. See Circle 1. E. coli colonies appear blue tored-blue and produce gas, confirm blue to red-blue colonies without gas. See Circle 2.

In some validation processes, this interpretation has been modified : the following organizations AOAC, NMKL, EMMAS have validated or assessed the use of the Petrifilm EC plates under specific conditions. See pages 2 and 3 of this Interpretation Guide. Remark : Most E. coli O157:H7 strains are atypical : theydo not grow at temperatures ≥ 44.5°C, are glucuronidase negative, and therefore will not produce a blue precipitate. They will appear as non-E. coli Coliforms (red with gas).

Coliforms:
The Petrifilm EC plates can also be used to search for Coliforms. • ISO defines Coliforms by their ability to grow in method-specific, selective media. ISO method 4832, enumerating Coliforms by the colony counttechnique, defines Coliforms by colony size and acid production on VRB with lactose (VRBL) agar. On Petrifilm EC plates, these acid-producing Coliforms are indicated by red colonies with or without gas (within approximately one colony diameter). See Circle 3. ISO method 4831, enumerating Coliforms by the Most Probable Number (MPN) method, defines Coliforms by their ability to grow and produce gasfrom lactose in a selective broth. On Petrifilm EC plates these Coliforms are indicated by red colonies associated (within approximately one colony diameter) with gas. See Circle 4. • AOAC INTERNATIONAL and U.S. FDA Bacteriological Analytical Manual (BAM) define Coliforms as Gramnegative rods which produce acid and gas from lactose during metabolic fermentation. Coliform colonies growing on thePetrifilm EC plate produce acid which deepen the gel color. Gas trapped around Coliform colonies (within approximately one colony diameter) indicates confirmed Coliforms. See Circle 4.

Interpretations of 3M Petrifilm EC Plates
TM TM

Recommended method in France Incubation: 24h +/- 2h at 42°C +/- 1°C Interpretation: • E. coli : Count all blue colonies with and without gas.

1A
53 E. coli.

Readingfollowing AOAC®. International all foods (method 991.14)
Incubation: • Coliforms in all foods: incubate 24h +/- 2h at 35°C +/- 1°C. • Enumeration of E. coli in all foods, except those here under: incubate 48h +/- 2h at 35°C +/- 1°C.

Reading following AOAC® International, meat, poultry and seafood (method 998.08)
Incubation: • Enumeration of E. coli in Meat, Poultry and Seafood, and Coliforms in...