1- By plating 0.1ml of inoculum we got 26 colonies. So the CFU/ml is 260 in the diluted culture. Given that the culture is diluted by 10-9 times before plating on to the plate. SO the initial CFU in the culture will be 260*109 or 2.6*1011CFU/ml.
A- culture must not have mixed properly in the diluent before plating on the plate.
B- CFU/ml in the initial culture must have been different in both the culture. So we got the different number of colonies on the plate.
4- Milk will only be safe if the CFU/ml is less than 500cfu/ml according to American Public Health Association. So if we get less than 5 colonies on the plate then we can consider the milk to be safe for drinking
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