Question

19 The original cell ensity in a sample is 2.79 X 10*CFUmL. Which sample volone should yield a countable plater (Express your answer as 10* ml)

Answer 1

19. Original density= 2.79 X10⁶ CFU/mL

Hence, 1 mL= 2.79 X 10⁶ cells

0.1 mL= 10^-1 mL= 2.79 X 10⁵ cells

0.01 mL= 10^-2 mL= 2.79 X 10⁴ cells

0.001 mL= 10^-3 mL= 2.79 X 10³ cells= 2790 cells

0.0001 mL= 10^-4 mL= 2.79 X 10² cells= 279 cells

0.00001 mL= 10^-5 mL= 2.79 X 10 cells= 27.9 cells

Most reports give Countable number of colonies are between 30-300 colonies on the plate, which is statistically significant.

0.0001 mL will give 279 colonies which is countable.

Sample volume giving countable number of colonies= 10^-4 mL (279 colonies)

However, as per FDA, countable number of colonies are 25-250. In this case,

Sample volume giving countable number of colonies= 10^-5 mL (27.9=28 colonies)

Dilutions need to be performed of the original samples, as such small sample volume cannot be pipetted.

Answer 2

Hence, 1 mL= 2.79 X 106 cells

0.1 mL= 10-1 mL= 2.79 X 105 cells

0.01 mL= 10-2 mL= 2.79 X 104 cells

0.001 mL= 10-3 mL= 2.79 X 103 cells= 2790 cells

0.0001 mL= 10-4 mL= 2.79 X 102 cells= 279 cells

0.00001 mL= 10-5 mL= 2.79 X 10 cells= 27.9 cells

Most reports give Countable number of colonies are between 30-300 colonies on the plate, which is statistically significant.

0.0001 mL will give 279 colonies which is countable.

Sample volume giving countable number of colonies= 10-4 mL (279 colonies)

However, as per FDA, countable number of colonies are 25-250. In this case,

Sample volume giving countable number of colonies= 10-5 mL (27.9=28 colonies)

Dilutions need to be performed of the original samples, as such small sample volume cannot be pipetted.