Petrifilm® 3M

3M Petrifilm E. coli Count Plate

To be used for the enumeration of Escherichia coli and coliforms.


The 3M Petrifiim E. coli Count Plate is a reliable, sample-ready medium system for enumerating Escherichia coliand other coliforms. Petrifilm E. coli Count plates contain violet red bile nutrients, a cold water soluble gelling agent, a glucuronidase indicator to identify E. coli, and a tetrazolium indicator to enhance the visualization of other gram coliform negative (non-E. coli) bacteria.

Coliforms ferment the lactose in the medium to produce gas. This gas is trapped around the coliform colony and allows the differentiation of coliform bacteria from other gram negative bacteria. In addition, glucuronidate, produced by most E. coli will react with the glucuronidase indicator In the medium to produce a blue precipitate around the colony allowing visual identification of E.coli.

Warning: Petrifilm E. coil Count plates are not intended to be used in the diagnoses of conditions in humans or animals.

Storage and disposal:

Store unopened Petrifilm plate foil pouches at or below 80C (460F). After opening, return unused plates to foil pouches. Seal foil pouch by folding and taping the open end. Store resealed pouches at room temperature (<25C or <77F) with <50% RH. Exposure of Petrifilm plates to higher temperatures and/or humidities can affect the performance of the plates. Do not refrigerate opened packages. Use Petrifilm plates within one month after opening. Do not use plates that show orange or brown discoloration.

After use, Petrifilm E. coil Count plates will contain viable bacteria. Follow current Industry standards for disposal.

Directions for use:

1. Place the Petrifilm E. coil Count plate on a flat surface (see figure A).

2. Lift top film and dispense 1 ml of sample onto the center of the bottom film (see figure B).

3. Slowly roll the top film down onto the sample to prevent the entrapment of air bubbles (see figure C).

4. Distribute sample evenly within the circular well using a gentle downward pressure on the center of the plastic spreader (flat side down) (see figure D). Do not slide spreader across the film. Remove spreader and leave plate undisturbed for one minute to permit solidification of the gel.

5. Incubate plates in a horizontal position with the clear side up in stacks not exceeding 20 plates. Follow current industry standards for incubation temperature. Incubate plates 24 + 2 hr. and examine for coliform and E. coli growth. Some E. coilcolonies require additional time to form the blue precipitate. Re-incubate plates an additional 24 + 2 hr. to detect any additional E. coil growth.

6a. Petrifilm E. coil Count plates can be counted on a standard colony counter. The Interpretation of E. coli colonies on Petrifilm E. coil Count plates varies by method. When considering which interpretation to follow please keep in mind that approximately 95% of E. coil produce gas.

AOAC Method (991.14)

Petrifilm plates incubated at 35 + 1C
Blue colonies associated with entrapped gas (see figure F) are confirmed E. coli. Blue colonies without gas are not counted as E. coil.

Other coliform colonies will be red and associated with gas bubbles. Colonies not associated with gas (with a distance greater than one colony diameter from gas bubble) are not counted as coliforms. The total coliform count consists of both the red and blue colonies associated with gas at 24 hours.

AFNOR Method (01/4 - 09/92)

Petrifilm plates incubated at 44.5 + 0.5C.
Blue colonies with gas are E. coil. Blue colonies without gas may or may not be E. coil and may be confirmed if necessary.

*Please note that E. coli 0157:H7 does not grow at > 44.5C and is glucuronidase negative and therefore will not produce a blue precipitate. For further information on identification of E. coil 0157:H7 please refer to the 3M Petrifilm Test Kit-HEC.

Do not count colonies on the foam dam since they are removed from the selective influence of the medium.

The circular growth area is 20 cm2. Estimates can be made on plates containing greater than 150 colonies by counting a representative number of squares and multiplying by the appropriate number to obtain an estimated count for the total 20 cm2 growth area.

6b. High concentrations of E. coli will cause the growth area to mm a bluish color (see figure F), while high concentrations of coliforms (non-E. coli) will cause the growth area to turn a dark reddish color (see figure G). When this occurs, further dilution of the sample is required to obtain a more accurate count.

7. To isolate colonies for further identification, lift the top film and pick the colony from the gel (see figure H).

Refer to the "Interpretation Guide" and "Reminders for Use" for more detailed information. For additional technical information, please contact your local subsidiary.

Coliform - E. Coli Petrifilm