17. Jason had to calculate the dilution factor in order to plate serial dilution

Question

17. Jason had to calculate the dilution factor in order to plate serial dilutions his concentrated cells as the correctly plating volume. He wanted his experiment to be consistent with his previous results. He saw that his starting concentration was 10 million cells/mL. The volume of the he use before dilution was 1 mL and the volume that he used after the dilution was 20 L. a) What is the dilution factor he used? Show your work. (5 points) Reneè was asked by a colleague to calculate the dilution factor of an antibiotic that was used for their bacterial culture. The antibiotic stock was 10 mg/mL which was used at a 50 g/mL. a) What is the dilution factor? (5 points) 8. How much of the stock solution does Reneè need to add to a 250 mL culture? Show your work. (5 points) b)

Explanation / Answer

Answer to question 17

Let us summarize the given information:

1. Starting concentration of cells = 10 x 106 cells per ml

2. Volume of the culture he used before dilution was 1 ml

3. Volume used after dilution was 20 microlitres

First, we have to calculate the final volume Vf

Vf = aliquot volume (Vi) + diluent volume (Vd)

For the first dilution, he used 1 ml of the concentrated culture. Generally, in serial dilution, 1 ml of concentrated culture will be diluted in 9 ml of diluent.

So,

Vf = aliquot volume (Vi) + diluent volume (Vd)

Vf = 1 ml of culture + 9 ml of diluent = 10 ml

The dilution factor (DF) for the first serial dilution can be calculated as follows:

DF = Vf / Vi = 10/1 = 10

Thus in the first serial dilution, the culture was diluted by a factor of 10

Now let us calculate the dilution factor for the aliquot used for plating. In the question, it is mentioned that 20 microlitre of the culture (0.02 ml) was used after dilution.

So,

DF = Vf / Vi = 10 / 0.02 = 500

The culture is diluted by a factor of 500.

Prior to this dilution, there was another dilution and the dilution factor was 10.

The final dilution factor he used to plate serial dilution is 500 x 10 = 5000

Answer to question 18

a) Dilution factor of the antibiotic used for the bacteria culture is

DF = Stock concentration / Final concentration

DF = (10 mg/ml) / (0.05 mg/ml)

DF = 200

So, the dilution factor of the antibiotic used for the culture is 200

b) The given problem is based on the following formula:

V1C1 = V2C2

Where,

V1 = ?

C1 = 10 mg/ml

V2 = 250 ml

C2 = 0.05 mg/ml

V1C1 = V2C2

V1 x (10) = 250 x 0.05

V1 = (250 x 0.05) / 10

V1 = 1.25 ml

So, Renee has to add 1.25 ml of antibiotic from the stock solution and 248.75 ml of culture. This will give 50 microgram/ml concentration of antibiotic in 250 mL of culture.

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