Define the Petrifilm (dry film) Method?

An alternative method to the conventional SPC is a Petrifilm (dry film) method. It is a non-petri dish plating system where a layer of nutrient gel is present on a paper backing covered by a thin flexible film. Indicator dye - tetrazolium- is present which imparts red colour to the colony. All red spots are counted as coloues. Petri films can also be used for detection and enumeration of specific groups such as coliforms.

Another method uses spiral plater, which is a mechanical device that distributes the liquid inoculum on the surface of the rotating agar medium plate. Special counting grid is used for enumeration of colonies. Here less agar is used as compared to standard plating and also more samples per hour can be examined. The method is, however, more suited for liquid foods.