Question

In many states, the legal limit for all bacteria present in pasteurized milk is not to exceed 20,000 CFU/mL. You have been away on vacation and left an unopened plastic jug of milk in your refrigerator. The "Sell By" date was three days ago. Instead of throwing it out, you decide to test it using your eScience Microbiology Kit. Here are your results for a "countable" plate in your serial dilutions: 186 colonies on a plate where the dilution factor was 1 x 10-4 and the volume that you plated was 1 mL. (20 points)
a. How many bacteria are present in 1 mL of this milk?
b. Is this milk safe to drink by Public Health standards?

Answer 1

Answer:

a. The formula for finding the number of colonies in a given volume is,  

CFU/ml, here CFU is the colony forming units or the number of colonies formed in a volume plated in ml × dilution

The dilution factor given in the question is 1 × 10⁻⁴ and the number of colonies given is 186. Therefore, the number of bacteria present in the given 1 ml of the milk is,  

CFU/ ml = 186 / 1 × 10⁻⁴ = 186 × 10⁴.  

Hence, the concentration of bacteria in the milk is 186 × 10⁴ CFU/ml.  

b) The limit of the bacterial concentration in the pasteurized milk is 20000 CFU/ml, however, it can be seen that the sample of the milk possess more concentration of bacteria than the desired value. Therefore, the milk is not safe to drink by Public Health Standards.