Micro December 2018

down onto the sample and the 3M Petrifilm Flat Spreader was used to spread the sample 1

evenly over the plate. For comparison to BAM Ch. 4, the 3M Petrifilm Rapid E. coli /Coliform 2

Count Plates were incubated for 18–24 h at 32 ± 1°C for dairy products and 35 ± 1°C for all 3

other matrices. For comparison to the ISO methods, the 3M Petrifilm Rapid E. coli /Coliform 4

Count Plates were incubated for 18–24 h at 30 ± 1°C (dairy) and 37 ± 1°C (all other matrices) for 5

ISO 4832:2006 and at 42 ± 1°C for ISO 16649-2:2001. Following incubation, the colonies were 6

observed through the clear side of the 3M Petrifilm Rapid E. coli /Coliform Count Plate. Each 7

blue to blue-green colony with or without gas production was represented as one E. coli CFU, 8

while each red colony with gas production within one colony diameter was represented as one 9

coliform CFU. The sum of the colonies was reported as the total coliform CFU/g of the diluted 10

sample with the E. coli CFU/g being reported separately. Counts less than or equal to 100 11

CFU/plate were considered countable. For the purposes of this validation study only, duplicate 12

3M Petrifilm Rapid E. coli /Coliform Count Plates were prepared for each sample dilution of each 13

method comparison. Duplicate plate counts were averaged, and then the average count was 14

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reported.

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Coliform colonies were confirmed by transferring a minimum of 10 presumptive colonies

from the 3M Petrifilm Rapid E. coli /Coliform Count Plate to BGLB broth and incubating at 32 ± 17

1°C for dairy products and 35 ± 1°C for all other matrices. Tubes were examined at 24 and 48 h 18

for gas production. For E. coli , a minimum of five colonies were confirmed by transferring a 19

presumptive colony from the 3M Petrifilm Rapid E. coli /Coliform Count Plate to EC broth and 20

incubating for 24 ± 2 h at 44.5 ± 0.2°C and then examining for gas production. Tubes negative 21

for gas at 24 h were reincubated for a total of 48 ± 2 h. Gas-positive tubes were streaked on 22

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