Carolina 3M Petrifilm E.Coli/Coliform Count Plates - Manual (Page 8)

4. To inoculate the medium, lift the top film. The circular gel area will adhere
to the top film (fig. 10). Tape the Petrifilm plate to a flat surface in the open
position for streaking. Streak with a sterile inoculating loop more gently
and with less pressure than you would use on a standard agar plate.
Figure 10
5. Incubate the plates with the gridded side down, in stacks of up to 20
plates.
Colonies may be isolated for further study or to inoculate additional
cultures. Lift the top film and pick the colony from the gel (See Method A,
step 10).
*Do not use diluents that contain citrate or sodium thiosulfate. These
substances are not found in common microbiological media such as Luria
broth or nutrient broth.
Interpretation
The U.S. Food and Drug Administration Bacteriological Analytical Manual
defines coliforms as gram-negative rods that produce acid and gas from
lactose fermentation. E. coli is a coliform, but unlike other coliforms, most
strains of E. coli produce the enzyme beta-glucuronidase. Petrifilm EC
Plates are formulated to differentiate non-coliform, coliform, and E. coli
colonies.
Petrifilm EC Plates contain an indicator dye in the medium that stains
bacterial colonies red. The red color helps to distinguish them from dust
particles or other environmental contaminants.The EC Plate growth
medium contains lactose, and the top film of the plates traps gas produced
by coliforms. Finally, EC Plates contain an indicator of beta-glucuronidase
activity, 5-bromo-4-chloro-3-indolyl-beta-D-glucuronide (BCIG). The enzyme
beta-glucuronidase cleaves BCIG to produce a blue product that precipates,
causing a beta-glucuronidase-producing bacterial colony (such as E. coli) to
appear blue. With this formulation, non-coliform colonies growing on
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