Carolina 3M Petrifilm Aerobic Count Plates - Manual (Page 6)

8. Incubate plates with the clear side up in stacks of up to 20 plates.
Incubation time and temperature will vary according to the application
and equipment available.
9. Colonies on Petrifilm plates can be counted on a standard colony
counter or other light source (fig.8). Bacterial colonies on Petrifilm AC
plates are red because of the indicator dye in the medium.
Figure 8 Figure 9
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.
11. Disinfect before disposal. Petrifilm plates can be disinfected by
autoclaving or by soaking in 20% bleach for one hour. Then, they can
be placed in the trash. Open the plates in the bleach to expose
organisms on the plates to the solution. 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 plate. These substances are not found in
common microbiological media such as Luria broth or nutrient broth.
Method B. Hydrating and using as solid medium
Hydrate and spread each Petrifilm plate before going on to the next plate.
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.
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