A sample of raw milk is diluted as follows:20.0 mL of milk is transferred to an

Question

A sample of raw milk is diluted as follows:20.0 mL of milk is transferred to an 80.0 mL sterile water blank and mix thoroughly.0.20 mL from this tube is transferred into a 3.80 mL sterile water blank and mixed thoroughly. Then 0.10 mL from this container is transferred to another tube which contains 9.90mL of sterile water and mixed thoroughly. From this container 1.00 mL is transferred into a 9.0 mL sterile water. Draw a picture of the process just described showing exact amounts. For each dilution, identity the dilution factor as a fraction, ratio or decimal. From each of the dilution you inoculate a plate of media with 0.10 ml of each dilution and 20 ml of sterile agar. On the plate with the third plate you count 137 colonies. Using this information estimate the number of cfu/mL in the original sample of raw milk. Show work below and place the correct answer in the given blank.

Explanation / Answer

Answer:

52. First dilution = 20/100 = 1/5

Second dilution = 0.2/4 * 1/5 = 1/100

Third dilution = 0.1/10 * 1/100 = 1 * 10-4

Fourth dilution = 1/10 * 10-4 = 10-5

53. Number of colonies on the third plate = 137

Dilution on the third plate = 10-4 * 1/10 (0.1 mL has been plated)

= 10-5

So, cfu/mL in the original sample = 137/10-5 = 137 * 105 = 1.37 * 107 cfu/mL

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