3 Petrifilm™
E. coli Count Plate Results from Most Probable Number (MPN) Results Conversion Table
Conversion Table
The 3M™ Petrifilm E. coli Count (EC) Plate method meets the criteria of several major global validating organizations.1 Although the Petrifilm EC Plate count cou nt method is an acceptable alternative to the Most Probable Number (MPN) method, laboratories occasionally have the need to determine Petrifilm EC Plate counts from MPN results. These laboratories have the MPN results for a food sample and are interested in determining the Petrifilm EC Plate count. Based on several studies, Table 1 was developed to show the relationship between MPN results and Petrifilm EC Plate counts. counts in flour, peanut butter, nut pieces, piece s, pepper, sausage, ground pork, beef with gravy, gravy, shrimp, frozen potatoes, macaroni, liquid eggs, lettuce, and raw milk were were recorded by an independent laboratory during an AOAC® Precollaborative Study (unpublished) using the Petrifilm EC Plate method and the MPN method. Three hundred forty-eight samples samples were used in the analysis. The relationship between between the two methods is described by the following equation:
Escherichia Escherichia coli coli
(log Petrifilm EC Plate result) = 0.37 + 0.90 * (log MPN result)
Using this equation, the Petrifilm EC Plate results can be calculated from a known MPN result. Table 1 conveniently conveniently lists Petrifilm EC Plate results from many known MPN sequences. Simply read across the row in order to determine the Petrifilm EC result from the MPN result. Since one study cannot represent all foods and all situations, each food o peration needs to verify the relationship for its specific application.
1.
Official Recognitions for the Petrifilm E. coli Count Plate: AOAC Official Methods of Analysis, USA: Method # 991.14 Health Protection Branch (HPB), Canada: Method MFHPB-34 Nordic Committee Committee on Food Analysis Analysis (NMKL): (NMKL): Method NMKL NMKL 147.1993 Venezuelan Commission of Industrial Standards: Covenin 3276:1997
3 Microbiology
Table 1. MPN to Petrifilm EC Plate Results Conversion Table MPN index (starting at 1:10 dilution)
MPN Sequence 0 0 0 0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
0 0 1 1 2 3 0 0 0 1 1 2 2 3 0 0 0 1 1 1 2 2 2 3 3 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3
0 1 0 1 0 0 0 1 2 0 1 0 1 0 0 1 2 0 1 2 0 1 2 0 1 0 1 2 0 1 2 3 0 1 2 3 0 1 2 3
Petrifilm EC Count Plate result
<3.0 3.01 3.05 6.11 6.19 9.44 3.57 7.23 11 7.36 11.2 11.4 15.4 15.7 9.18 14.3 19.9 14.7 20.5 26.8 21.1 27.6 34.8 28.6 36 23.1 38.5 63.6 42.7 74.9 115 159 93.3 149 215 292 240 462 1100 >1100
<6.0 6 6 12 12 18 7 14 20 14 21 21 27 28 17 26 35 26 36 45 36 46 57 48 59 40 63 98 69 114 168 225 139 212 295 388 325 586 1280 >1280
Note: MPN results based on a 3 tube MPN starting at a 1:10 dilution (i.e., 3 tubes at 1:10, 3 tubes at 1:100 and 3 tubes at 1:1000).
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