Enumeration of E. coli and Coliforms in Food Products and Food Ingredients Using 3M Petrifilm E. coli Plates

3.3 E. coli and other coliforms form gas from the fermentation of lactose in the medium. The gas is trapped by the films and appears as a small bubble(s) associated with a red or blue colony on the plate.

3.4 Plastic spreading devices are provided with the Petrifilm plates. The smooth flat side is designed to spread the sample over the growth area.

4. COLLECTION OF SAMPLES

See Appendix B of Volume 2.

5. MATERIALS AND SPECIAL EQUIPMENT

1) Petrifilm EC plates 6404/6414 (3M Canada Inc., PO Box 5757, London, ON, N6A 4T1)

2) Appropriate diluents: Butterfield's phosphate buffer (7.9), IDF phosphate buffer (7.13), 0.1 % peptone water, peptone salt diluent (ISO method 6887), 0.85 - 0.9% saline solution, bisulfate-free letheen broth or distilled water. If citrate buffer is indicated in the standard, substitute warmed (40^o - 45 ^oC) Butterfield's phosphate buffer

Note: Do not use citrate buffer or sodium thiosulfate with the Petrifilm method.

3) Stomacher, blender or equivalent

4) Incubator capable of maintaining 35^oC

Note: It is the responsibility of each laboratory to ensure that the temperature of the incubators or water baths are maintained at the recommended temperatures. Where 35^oC is recommended in the text of the method, the incubator may be at 35 +/-1.0^oC. Similarly, lower temperatures of 30 or 25^oC may be +/- 1.0^oC. However, where higher temperatures are recommended, such as 43 or 45.5^oC, it is imperative that the incubators or water baths be maintained within 0.5^oC due to potential lethality of higher temperatures on the microorganism being isolated.

5) pH meter or paper capable of distinguishing 0.3 to 0.5 pH units within a range of 6.0 to 7.5

6) 1N NaOH

7) colony counter and/or magnified illuminator

6. PROCEDURE

Carry out the test in accordance with the following instructions:

6.1 Handling of Food Sample