Micro December 2018

broth and incubating at 35 ± 1°C for 48 ± 2 h. Tubes were then examined for fluorescence. In 1

addition, Gram stain and IMViC tests were conducted on a minimum of one presumptive E. coli 2

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colony per sample.

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ISO 4832:2006

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All matrices were evaluated in a paired study design, and so all samples prepared for the 7

3M Petrifilm Rapid E. coli /Coliform Count Plate analysis in PSS were also used for the ISO 8

4832:2006 analysis. A 1.0 mL aliquot from each sample dilution was plated in duplicate onto 9

Petri dishes. To each plate, 15 mL of VRBL agar was added, swirled clockwise, then 10

counterclockwise, taking care to avoid spillage on the Petri dish lid and allowed to solidify. Once 11

solidified, an overlay of about 4 mL of VRBL agar was poured onto the surface of each plate. All 12

plates were inverted, incubated at 30 ± 1°C for dairy and 37 ± 1°C for all other matrices for 24 ± 13

2 h and enumerated. The duplicate plates for each sample dilution were averaged, and then the 14

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average count was reported.

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Counts between 10 and 150 CFU/plate were considered countable, while counts outside

that range were considered estimates. Purple-red colonies that were 0.5 mm or larger in 17

diameter and sometimes surrounded by a zone of precipitated bile acids were counted as 18

coliform colonies. These colonies did not require additional confirmation. Atypical coliform 19

colonies were enumerated as well, and 5 colonies of each atypical type were picked to a tube 20

containing BGLB broth and incubated at 37 ± 1°C for 24 ± 2 h. After incubation, tubes were 21

examined and colonies that contained gas production were confirmed as coliform. 22

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