Petrifilm Environmental Listeria Interpretation Guide - NOACK

3M Petrifilm Interpretation Guide3M Petrifilm Environmental Listeria PlatesThis guide familiarizes you with results on 3M Petrifilm Environmental Listeria Plates. For more information, contact the 3M Microbiologyrepresentative nearest you.The Petrifilm environmental Listeria (EL) plate is a sample-ready culture medium containing selective agents, nutrients, a cold-watersoluble gelling agent and a chromogenic indicator that facilitates Listeria colony detection. Petrifilm EL plates are designed to analyse environmentalsamples and to help increase the efficiency of monitoring plant sanitation. The presence of indicator Listeria such as Listeriainnocua provides evidence that environmental conditions are suitable for the occurrence of Listeria monocytogenes. The Petrifilm EL platedetects the majority of environmental Listeria, consisting of Listeria monocytogenes, Listeria innocua, and Listeria welshimeri.*Many organisms in the environment can be stressed by environmental conditions or sanitizers. Buffered peptone water (BPW) is used as arepair broth in conjunction with the Petrifilm EL plate to resuscitate stressed Listeria without increasing their numbers. Repair in BPW is notan enrichment step.1.1.This Petrifilm EL plate has NO colonies after 28h of incubation.The test is complete.Quantitative Interpretation: Listeria colonies on this plate:

QuantitativeSampling & InterpretationIf your facility chooses to use the Petrifilm environmental Listeria plate in a quantitative manner, please refer to the product packageinsert, and then calculate the colony forming units (CFU) per area as shown below. You may also want to consider the following points:• Consistency is the key to obtaining useful information from your environmental monitoring programme. Use a consistent procedure each time that you sample.Ideally, use the same type of sampling device and techniques.• The sampling area size may be based on regulations, internal standards and/or the location of the monitoring. For example, you may need to sample a largerarea for a finished goods area because the numbers of bacteria are expected to be low.• More information on environmental sampling can be found in the references listed below and in the Petrifilm plates environmental monitoring proceduresbrochure.TO DETERMINE the quantity of Listeria per sampled area, you will need to record:1) area size sampled2) volume of hydration fluid in the sampling device3) volume of the buffered peptone water added4) volume plated5) number of colonies countedAPPLY the following equation or worksheet to determine the CFU/area sampled. Examples are given on the following pages.See Package Insert & Reminders for Use for full details of the method.You may also determine the result per sample, e.g., CFU/ drain.CFU/area = (Number of colonies x [mL hydration fluid + mL BPW] ÷ 3 mL) ÷ area sampledorA. Total number of mL of BPW + hydration fluid:B. Number of mL plated:C. Divide line A by line B:D. Number of colonies counted:(if number of colonies is zero, insert “

Quantitative InterpretationExample: Swab Contact Method31 2 water (see line A).Using a swab (or equivalent) moistenedwith 1 mL of hydration fluid(see line A), sample an area. For thisexample, area is fifty square centimeters(50 cm 2 ) (see line F). Returnswab to sterile container.Add 2 mL of buffered peptoneAfter repair step, plate 3 mLonto the Petrifilm environmental3Listeria plate (see line B).After incubation, count colonies.4 For this example, assume youcount fifty (50) colonies (seeline D).A. Total number of mL of BPW + hydration fluid:B. Number of mL plated:C. Divide line A by line B:D. Number of colonies counted:E. Multiply line C by line D:F. Area sampled:G. Divide line E by line F:1 + 2 = 3A3B1C50D50E50 cm 2F1 CFU/cm 2 GExample: Sponge Contact Method3Using a sponge moistened with 101 mL of hydration fluid, sample anarea (see line A). For this example,area is fifty square centimeters(50 cm 2 ) (see line F).Return the sponge to the sterile2 container and add 5 mL of bufferedpeptone water (see line A).After repair step, plate 3 mL ontothe Petrifilm environmental Listeria3plate (see line B).After incubation, count colonies.4 For this example, assume youcount ten (10) colonies (seeline D).A. Total number of mL of BPW + hydration fluid:B. Number of mL plated:C. Divide line A by line B:D. Number of colonies counted:E. Multiply line C by line D:F. Area sampled:G. Divide line E by line F:10 + 5 = 15A3B5C10D50E50 cm 2F1 CFU/cm 2 G

Additional Comments3M Microbiology offers a full range of Petrifilm count plates designed to meet microbial testing requirements withinthe Food Industry.For further product information please visit our website:www.3M.com/microbiology3M Deutschland GmbH3M MicrobiologyCarl-Schurz-Straße 141453 NeussGermanyPhone +49 (0) 2131/14 4350Fax +49 (0) 2131/14 4397Internet www.3m.com/microbiologyPlease recycle. Printed in Germany.© 3M 2008. All rights reserved.1389-101-EU3M and Petrifilm are trademarks of the3M company.