Micro December 2018

not typical coliform strains. However, ISO 4832:2006 defines coliforms by their ability to grow 1

on specific, selective media, so gas production is not a requirement for coliform identification. 2

Because this validation was conducted in harmonization with MicroVal, on the 3M Petrifilm 3

Rapid E. coli /Coliform Count Plate, coliforms are indicated by red colonies with or without gas 4

production and blue colonies with or without gas production. By this definition, the non-gassing 5

Citrobacter and Serratia are considered positive as coliforms. All other non- E. coli coliform 6

isolates tested were positive (red colonies with gas production), and the E. coli isolates tested 7

were positive (blue to blue/green). All non-coliform strains tested were negative (Tables 5–8). 8

9

In the matrix study, 31 foods and two environmental surfaces were tested by the 3M

Petrifilm Rapid E. coli /Coliform Count Plate in comparison to BAM Ch. 4 and 12 foods and two 10

environmental surfaces in comparison to ISO 4832:2006 and ISO 16649-2:2001. Mean 11

differences of log transformed results between the 3M Petrifilm Rapid E. coli /Coliform Count 12

Plate and the reference methods were well below 0.5 log 10 for all contamination levels of all 13

matrices tested. Differences were <0.2 log 10 and lower in most cases when comparing the 3M 14

Petrifilm Rapid E. coli /Coliform Count Plate to BAM Ch. 4 and <0.06 log 10 when comparing to 15

the ISO methods. The same contaminated materials were used for each method, so the smaller 16

differences with the ISO methods could be due to the test portion sizes (50 g for the BAM 17

method and 10 g for the ISO methods), the different diluents (BPBD for BAM and PSS for ISO) or 18

more likely the different media compositions. Also, foods with larger mean differences, such as 19

butter, non-fat dry milk powder, shell eggs, liquid egg whites, dry dog kibble and dry cat food 20

were only tested in the BAM comparison and were not tested in the ISO comparison. For these 21

foods, the recovery of coliforms and E. coli was generally higher for the 3M Petrifilm Rapid E. 22

27