Carolina 3M Petrifilm Enterobacteriaceae 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
Bacterial colonies on Petrifilm EB plates are red because of an indicator dye
in the medium. The red color helps to distinguish them from dust particles
or other environmental contaminants. An additional indicator in the
medium shows whether acid is produced by the organisms growing there.
A low-oxygen environment is required for organisms to ferment sugars to
acid and thus obtain the colored haloes described below. Inoculation
Method A ensures that organisms growing on EB plates will be exposed to
limited oxygen. Inoculation Method B may permit more oxygen to reach
the organisms.
The EB plates contain the sugar glucose and a purple-blue indicator dye.
Enterobacteriaceae (the group including E. coli and other coliforms) are
defined as gram-negative rods that ferment glucose to produce acid and/or
gas. If the organisms can ferment glucose, the colonies will be red with
surrounding yellow zones (indicating acid production) and/or gas bubbles.
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