3M™ Petrifilm™ Select E.coli Count Plate

3M™ Petrifilm™ Select E. coli Count Plates
Reminders
for use
For detailed WARNINGS, CAUTIONS, DISCLAIMER OF WARRANTIES / LIMITED REMEDY,
LIMITATION OF 3M LIABILITY, STORAGE AND DISPOSAL information, and INSTRUCTIONS
FOR USE see Product’s package insert.
Storage
1
Store unopened packages at ≤8°C (≤46°F).
Use before expiration date on package. In areas of high humidity where condensation may
be an issue, it is best to allow packages to
reach room temperature before opening.
2
To seal opened pouch, fold end over and tape
shut.
3
Store resealed packages in a cool dry place for
no longer than one month. Do not refrigerate
opened packages. Store resealed packages in
a freezer if the laboratory temperature exceeds
25°C (77°F) and/or the laboratory is located in
an area where the humidity frequently exceeds
50%.
Sample Preparation
4
Prepare a 1:10 or greater dilution. Weigh
or pipette food product into an appropriate
sterile container such as stomacher bag,
dilution bottle, or other sterile container.
5
Add appropriate quantity of one of the following
sterile diluents: peptone salt diluent (ISO 6887),
buffered peptone water (ISO 6887), 0.1% peptone water, K 2HPO4 (IDF 122C), Butterfield’s
phosphate buffer (IDF 122C phosphate buffer, KH2PO4 at 0.0425 g/L, adjust pH to 7.2),
bisulphite-free letheen broth, quarter strength
Ringer’s (IDF 122C), saline solution (0.850.90%), or distilled water.
6
Blend or homogenise sample as per current
procedure.
Adjust pH of the diluted sample to between
6.5 and 7.5 :
• for acid products, use NaOH 1N,
• for alkaline products, use HCl 1N.
Do not use buffers containing citrate, bisulphite or
thiosulphate; they can inhibit growth.
Inoculation
7
Place Petrifilm plate on level surface. Lift
top film.
8
With pipette perpendicular to Petrifilm plate,
place 1 mL of sample onto centre of bottom
film.
9
Carefully roll top film down to avoid trapping
air bubbles. Do not let top film drop.
Inoculation
10
With flat side down, place spreader on top
film over inoculum.
11
Incubation
Gently apply pressure on spreader to distribute inoculum over circular area. Do not twist or
slide the spreader.
12
Lift spreader. Wait at least one minute for gel
to form.
Interpretation
<70°F
13
Incubate plates with clear side up in stacks of
up to 20. Incubate for 24h ± 2h at 42°C ± 1°C.
It may be necessary to humidify the incubator
to minimise loss of moisture.
14
Petrifilm plates can be counted with a standard colony counter or other illuminated magnifier. Refer to the Interpretation Guide section
when reading results.
15
Colonies may be isolated for further identification. Lift top film and pick the colony from the
gel. Test using standard procedures.
3M Deutschland GmbH
3M Microbiology
Carl-Schurz-Straße 1
41453 Neuss
Germany
Phone
+49 (0) 2131/14 4350
Fax
+49 (0) 2131/14 4397
Internet
www.3m.com/microbiology
Please recycle. Printed in Germany.
© 3M 2008. All rights reserved.
1375-101-EU
3M and Petrifilm are trademarks of the
3M company.
3M™ Petrifilm™
Interpretation Guide
3M Petrifilm Select E. coli Count Plates
™
™
This guide is to familiarize you with results on Petrifilm Select E. coli count (SEC) plates. For more information, contact the 3M Microbiology
Products representative nearest you.
1.
2.
E. coli count: 97
E. coli count: 0
Petrifilm Select E. coli count plates allow specific
detection of E. coli. About 97% of E. coli strains produce
ß-glucuronidase which reacts with an indicator dye in the
plate to produce dark green to blue-green colonies.
Colonies other than E. coli are difficult to see because they are colourless to a
light grey-beige.
E. coli O 157 will not be detected on Petrifilm SEC count plates because
most E. coli O 157 strains are glucuronidase negative.
3M™ Petrifilm™ Select E. coli Count Plates
Counting range :
The counting range on Petrifilm Select E. coli count plates is 15-150 colonies. Counts greater than 150 colonies may be estimated or considered as too numerous to count
(TNTC). To obtain an accurate count, dilute the sample further.
3.
4.
1
E. coli count: 56
Estimated E. coli count ≈ 740
Do not count colonies on the foam dam because they are removed from the
selective influence of the medium.
See circle 1.
When colonies number more than 150 estimates can be made. Count the number of colonies in one or more representative squares and multiply the average
number by 20 to obtain the estimated count per plate.
5.
6.
E. coli count: TNTC
E. coli count: TNTC
When present in large numbers, E. coli may appear as small, indistinct colonies.
High concentrations of E. coli will cause the entire growth area to turn bluegreen.
3M™ Petrifilm™ Select E. coli Count Plates
Interference from food products :
Petrifilm Select E. coli count plates have been evaluated using samples from many, but not all foods.
Foods tested include certain fresh and frozen meats, vegetables and seafood; frozen prepared meals; and fresh, fermented and dry dairy foods. In a limited
number of cases, such as liver, the food may interfere with enumeration. To reduce such food interference, dilute the sample further.
8.
7.
1
E. coli count: 42
E. coli count: 21
E. coli colonies can easily be distinguished from food particles which are often
irregularly shaped and variable in size and colour.
Circle 1 shows nut particles.
Some dark foods may produce a coloured background that makes E. coli colonies less distinguishable. Further dilutions will lighten the background colour
making the E. coli colonies easier to count.
Figure 8 shows cocoa powder diluted 1:50.
9a.
9b.
1
1:10 dilution
1:100 dilution
Raw liver contains ß-glucuronidase which produces a blue-green background colour on the growth area on the plates, making the E. coli colonies less distinguishable. Further dilution will lighten the background colour making the E. coli colonies easier to count and will help to distinguish food interference from TNTC
plates that have confluent colonies (see figure 6). Artifact bubbles may result from improper inoculation of the plate or from trapped air from the sample. See
circle 1.
3M™ Petrifilm™ Select E. coli Count Plates
Variability in E. coli colony appearance :
Glucuronidase-positive E. coli colonies may vary in size, colour intensity and shape, depending on the strain itself, the food and the influence of external factors
such as testing protocols. The blue-green E. coli colonies may have gas bubbles associated with them.
10.
11.
E. coli count = 92
E. coli count = 75
Pale green colonies may result either from E. coli that are weak producers of
glucuronidase or from an interaction with food such as those containing high
acid or high sugar.
Figure 10 shows a highly acidic fermented dairy product.
Greenish-brown colour variability may appear with some foods.
Figure 11 shows a kidney sample.
12.
13.
1
E. coli count = 10
E. coli count = 25
E. coli colonies may have gas bubbles associated with them depending on the
E. coli strain and the food. Count all colonies with or without gas.
Smeared colonies may appear. See circle 1.
To minimise the production of smeared colonies spread immediately after inoculation, pressing gently on the centre of the spreader.