Question

3.) You have a 25ml culture with 20 micrograms/ml streptomycin and 0.1 mg/ml Ampicillin describe how you will feed this culture and double the concentration of each drug while keeping the volume constant. You have a stock solution of 50mg/ml streptomycin and 100 mg/ml ampicillin.

Answer 1

20ug/ml = 0.02 mg/ml steptomycin

Ampicillin= 0.1mg/ml

For calculating volume of stock required is C1*V1 = C2*V2

Where, C1 is stock concentration, V1 is stock volume and C2 is desired concentration and V2 is desired culture volume (here 25 ml)

For, 0.02 mg/ml streptomycin, 50*V1=0.02*25 = 0.01 ml (10ul) of stock

For, 0.04 mg/ml streptomycin, 50*V1=0.04*25 = 0.02ml (20ul) of stock

For, 0.1 mg/ml ampicillin, 100*V1= 0.1*25 = 0.025 ml (25ul) of stock

For, 0.2 mg/ml ampicillin, 100*V1= 0.2*25 = 0.050 ml (50l) of stock  

Since, the 10-50 ul is very less compared to 25 ml, the final volume of culture (25ml) will remain unchanged.