Question

1. You have plated a 0.1 ml inoculum from a 10^-9 DF broth. You counted 26 Colonies. How would you report this information.

2. What are two reasons why plates with the identical dilutions and inoculum volumes would have drastically different colony numbers? (Hint: think about inoculation numbers)

3. Design a dilution scheme for a raw milk sample. You transfer 10 microliters of the original sample into a tube containing 990 microliters. You plate 10 microliters and count X amount of Colonies. Depict this dilution scheme, making sure to include dilution factors, dilute the volumes etc. be sure to include DF of your tube and the OCD of the sample. Make sure your design allows for countable plates.

4. If you wanted to drink the milk from the sample in the previous question, would it be safe? What amount of CFU/ ml is deemed safe?

Answer 1

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.

2-

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.

3-

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