3M Petrifilm 6447 Product Instructions page 4

Plating
1. Place the 3M Petrifilm EL Plate on a flat, level surface.
2. Prior to plating, mix or vortex the collected sample again.
3. Draw 3 mL of liquid from the collected sample. For some sampling devices, such as sponges, squeeze the device to
release the liquid for plating.
4. Lift the top film and with the pipette perpendicular dispense 3 mL of sample suspension onto the center of bottom
film.
5. Roll the top film down onto the sample to prevent trapping air bubbles.
6. Gently place the 3M™ Petrifilm™ Large Square Spreader on the center of the 3M Petrifilm EL Plate to distribute the
sample evenly over the entire 3M Petrifilm EL Plate growth area.
7. Remove the spreader and leave the 3M Petrifilm EL Plate undisturbed for at least ten minutes to permit the gel to
form.
Incubation
Incubate the 3M Petrifilm EL Plates in a horizontal position with the clear side up in stacks of no more than 10 plates.
Incubate 3M Petrifilm EL Plates for 28 hours ± 2 hours at 35°C ± 1°C or 37°C ± 1°C. Several incubation times and
temperatures can be used depending on current local reference methods, some of which are listed in the "Specific
Instructions for Validated Methods" section. Incubation beyond the recommended time may yield ambiguous results.
Interpretation
1. The 3M Petrifilm EL Plates can be counted or interpreted using a standard colony counter or other illuminated
magnifier.
2. The circular growth area is approximately 42 cm
3. The 3M Petrifilm EL Plate can be used as a quantitative, semi-quantitative, or qualitative test.
a. For a quantitative test, count and record all red-violet colonies. Do not count colonies on the foam dam since
they are removed from the selective influence of the medium.
b. For a semi-quantitative test, record the results as high, medium or low based on the relative number of red-violet
colonies present. This designation of high, medium or low is dependent upon the sample location and individual
plant standards.
c. For a qualitative test, record the results of sample plated as positive (detected) or negative (not detected) based
on the presence or absence of red-violet colonies.
4. If 3M Petrifilm EL Plates have been incubated for the minimum time and they have pink and/or gray colonies,
re-incubate those plates for up to the maximum incubation time to ensure optimal color development. Count and
interpret as in step 3.
5. When colonies are present in large numbers, 3M Petrifilm EL Plates may have small, indistinct colonies and/or a
pink-brown color throughout.
a. For a quantitative test, record the results as too numerous to count (TNTC).
b. For a semi-quantitative test, record the results as high.
c. For a qualitative test, record the results as positive (detected).
6. Where necessary, colonies may be isolated for further identification. Lift the top film and pick the colony from the
gel. Test using standard procedures.
7. If the 3M Petrifilm EL Plates cannot be counted within 1 hour of removal from the incubator, they may be stored for
later enumeration by freezing in a sealable container at temperatures lower than or equal to negative 15°C (5°F) for
no longer than one week. Organisms may not be viable for further identification after plates have been frozen.
For further information refer to the "3M™ Petrifilm™ Environmental Listeria Plate Interpretation Guide". If you have
questions about specific applications or procedures, please visit our website at www.3M.com/foodsafety or contact
your local 3M representative or distributor.
Specific Instructions for Validated Methods
AOAC-RI Performance Tested Methods (PTM)
In an AOAC® RI PTM study, the 3M Petrifilm EL Plate method was found to be equivalent to or better than the average
log counts of the reference method.
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