Question

You have a stock solution of rifampicin at a concentration of 3.5 g/ml. You need to prepare 2L of TSA+Rifampicin with a final antibiotic concentration of 50ppm. How would you prepare this media with the desired antibiotic concentration? Show your calculations.

Answer #1

Hi,

I think the stock solution of rifampicin might be 3.5 mg/mL which is 3500 ppm.

Now, Given is

Stock solution of Rifampicin (C1)= 3.5 mg/mL= 3500ppm

the final antibiotic concentration to be achiieved (C2) = 50 ppm

The Final Volume (V2)= 2L or 2000mL

Using the formula, C1V1=C2V2, 28.57 mL of stock solution of rifampicin should be added to tryptic soya agar and the final dilution should be made 2L.

**Note:** Now if the intial stock solution is 3.5
g/mL for rifampicin which is 3,500 mg/mL (3,500,000 ppm). Yow will
have to make a 1000 fold serial dilutions of stock solution to
achieve a final concentration of 3500 ppm or 3.5 mg/mL. Then follow
the above mentioned calculation to prepare 50 ppm rifampicin
solution in 2L TSA.

To make serial dilutions,

1. Pipette out 100 mL of stock solution of rifampicin (3500 mg/mL) and add 900 mL of diluent. This would be Dilution A with concentration 350 mg/mL.

2. Pipette out 100 mL of Dilution A (350 mg/mL) and add 900 mL of diluent. This would be Dilution B with concentration 35 mg/mL.

3.Pipette out 100 mL of Dilution B (35 mg/mL) and add 900 mL of diluent. This would be Dilution C with concentration 3.5 mg/mL.

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