Method of Determining the Count of Aerobic Mesophilic Bacteria (Plate Count Method)

Since direct counting of the bacteria contained in or on the above mentioned products (2) is impossible, an indirect procedure must be used for their determination. For this purpose the product to be investigated is first mixed with a sterile physiological solution in order to separe out and suspend the bacteria (8.1.) - if necessary after comminution of the product by means of a suitable device according to 6.9.1. or 6.9.2. A decimal dilution series (8.2.) is prepared from this initial suspension. Aliquots of the dilution stages are transferred to Petri dishes and mixed with a culture medium which at first is still molten (8.4.) After solidification of the agar the individual bacterial cells are fixed and can multiply during incubation and form colonies (8.5.). The number of colonies is determined (9.1.) and given as the "number of bacteria per gramme of the sample" (9.2.). The accuracy of the method depends on the extent to which it is possible to separate completely the bacterial cells from the substrate, avoid damage to the cells during the necessary handling and obtain a good, even distribution of the cells in the culture medium. Accuracy is also enhanced if the bacterial count is carried out on several sub-samples (three or preferably five).