Micro December 2018

( b ) Enumerate blue to blue-green colonies with and without gas, regardless of size or 1 intensity of color, as confirmed E. coli . Note: Do not use this plate for the specific detection of 2 E. coli O157. Because most E. coli O157 strains are atypical, for example they are 3 glucuronidase negative, they will not produce a blue color, and therefore will be detected as 4 coliforms on 3M Petrifilm Rapid E. coli /Coliform Count Plates. A few strains of bacteria can 5 produce β-glucuronidase such as Shigella , Salmonella , Enterobacter, Citrobacter and 6 Klebsiella and will produce blue to blue-green colonies on the 3M Petrifilm Rapid E. 7 coli /Coliform Count Plate. Foods with high sugar content may increase the potential for gas 8 production from non-coliform Enterobacteriaceae. 9 ( c ) The interpretation of non- E. coli coliform colonies on the 3M Petrifilm Rapid E. 10 coli /Coliform Count Plates varies by reference method. For example: 1 ) BAM Chapter 4 defines 11

coliforms as Gram negative rods, which produce acid and gas from lactose during metabolic 12

fermentation. Non- E. coli coliform colonies on the 3M Petrifilm Rapid E. coli /Coliform Count 13

Plates are red and closely associated (within one colony diameter) with entrapped gas. Colonies 14

not associated with gas (a distance greater than one colony diameter between colony and gas 15

bubble) are not counted as coliforms. The total coliform count consists of both the red colonies 16

with gas and blue colonies with and without gas. 2 ) ISO defines coliforms by their ability to 17

grow in method-specific, selective media. ISO 4832 enumerates typical coliform colonies on 18

Violet Red Bile Lactose (VRBL) agar, with confirmation of atypical colonies. On the 3M Petrifilm 19

Rapid E. coli /Coliform Count Plates, non- E. coli coliforms are red colonies with and without gas 20

production. The total coliform count is indicated by red colonies with or without gas production 21

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and blue colonies with and without gas production.

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