How do we dilute a bacterial culture 50-fold, 100-fold, and 200-fold?

1 Answer
Jul 15, 2015

Answer:

You could do the dilutions in one step, or you might have to do serial dilutions.

Explanation:

There is a limit to how much you can dilute a sample in one step.

You probably wouldn't want to dilute by more than a factor of 100 in a single step, because that would give you a large volume of dilute solution.

Rather, you would use the serial dilution technique.

Remember the formula for dilution factor (#"DF"#):

#"DF" = V_f/V_i#, where

#V_i = "aliquot volume"# and

#V_f = "final volume" = "aliquot volume + diluent volume"#

(a) #"DF" = 50#

Here we can do a single dilution.

We can add 0.2 mL of sample to 9.8 mL of diluent (normal saline? cells will burst in distilled water because of osmotic pressure).

#V_f = "0.2 mL + 9.8 mL" = "10.0 mL"#

#"DF" = V_f/V_i = (10.0 cancel("mL"))/(0.2 cancel("mL")) = 50#

(b) #"DF" = 100#

Here, too, we can do a single dilution.

This time we add 0.1 mL of sample to 9.9 mL of saline.

#V_f = "0.1 mL + 9.9 mL" = "10.0 mL"#

#"DF" = V_f/V_i = (10.0 cancel("mL"))/(0.1 cancel("mL")) = 100#

(c) #"DF" = 200#

Here you might want to do a serial dilution.

The formula for serial dilutions is

#"DF" = "DF"_1 × "DF"_2 × "DF"_3 ×…#

How about a 1:100 dilution followed by a 1:2 dilution?

(1) Add 0.1 mL of sample to 9.9 mL of diluent. #"DF"_1 = 100#. Call this "Solution 1".

(2) Add 5 mL of Solution 1 to 5 mL of saline. Call this "Solution 2".

#V_i = "5 mL"#

#V_f = "5 mL + 5 mL" = "10 mL"#

#"DF"_2 = V_f/V_i = (10 cancel("mL"))/(5 cancel("mL")) = 2#

#"DF" = "DF"_1 × "DF"_2 = 100 × 2 = 200#