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. Petrifilm CC Plates contain lactose, and the top film
traps gas produced by coliform growing on the plates. Coliform colonies
are identified by the gas bubbles associated with them. All bacterial
colonies on Petrifilm CC 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.
Sometimes gas bubbles are trapped when the inoculate is spread on
Petrifilm plates. These bubbles are irregularly shaped and not associated
with a bacterial colony, while gas bubbles produced by lactose-fermenting
bacteria are associated with a colony (see fig. 11.)
8
