Question

A typical concentration of ampicillin used for bacterial selection in liquid LB is 100 microliter/mL. we prepared a 100 mg/mL ampicillin stock solution for your use. if you will only use .5 mL of your prepared dilution to spread on a plate, what is a reasonable volume of each dilution to prepare? Do you really need hundreds of milliliters?

Answer 1

Ampicillin concentration in stock=100 mg/mL

0.5 ml of the solution should contain 100 microliter/mL of ampicillin as typical concentration of ampicillin used in bacterial selection is 100 microliter/mL

Hence, 1 ml of the solution should contain 200 microliter/mL of ampicillin

1mg= 1000 microliter

100 mg= 1000 x 100= 10⁵ microliter

100 microliter= 10² microliter =0.1 mg

1: 10 dilution of 100 mg/mL will give 10mg/ml, which is 10⁴ microliter/mL

1: 10 dilution of 10 mg/mL will give 1mg/ml, which is 10³ microliter/mL

1: 10 dilution of 1mg/mL will give 0.1mg/ml, which is 10² microliter/mL

Hence, the stock solution of 100 mg/ml should be diluted 1:10 X 1:10 X 1:10= 1: 1000 times to get 100 microlter/mL

The dilution can be combined as follows:

1 microliter of stock solution + 999 microliter of distilled water= 1 mL of 100 microliter ampicillin (100 microliter/mL)

In order to spread plate 0.5ml, the concentration in it should be 100 microliter.

You need to make a 200 microliter/mL solution, so that 0.5 mL of this solution contains 100 microliter ampicillin.

2 microliter of stock solution + 998 microliter of distilled water= 200 microliter/mL of ampicillin

Hence, 0.5 mL of this 200 microliter/mL will contain 100 microliter of ampicillin

The amount of ampicillin used is less, in few microliters. It is enough to prepare 1 ml of solution for one plate, as some volume is required in excess to prevent pipetting error.