7
10. Colonies may be isolated for further study or to inoculate additional
cultures. Lift the top film and pick the colony from the gel. The medium
will adhere to the top film (fig. 9).
Figure 9
11. Disinfect before disposal. Petrifilm plates can be disinfected by
autoclaving or by soaking in 20% bleach for 1 hour. Then, they can be
placed in the trash. Alternatively, they can be taken to a facility such as
a hospital or given to a school nurse for disposal with other
biohazardous material.
*Do not use diluents that contain citrate or sodium thiosulfate because they
can inhibit growth on the Petrifilm plates. These substances are not found
in common microbiological media such as Luria broth or nutrient broth.
Method B. Hydrating and using as solid medium
1. Follow steps 1–6 from Method A, using 1 mL of distilled water, liquid
culture medium, or buffers with pH between 6.6 and 7.2 to hydrate the
Petrifilm plate.* If antibiotics are to be added to the medium, add them
to the hydration liquid at the working concentration.
2. Lift the spreader. Wait at least 2 hours for the gel to solidify.
3. Hydrated Petrifilm CC plates can be stored in a sealed bag in the
refrigerator for up to 7 days before use.
