Question

A plaque assay is performed beginning with 1 mL of a solution containing bacteriophages. This solution is serially diluted 5 times by combining 0.1 mL of each sequential dilution with 9.9 mL of liquid medium. Then 0.1 mL of the final dilution is plated in the plaque assay and yields 30 plaques.

What is the initial density of bacteriophages in the original 1 mL? Recall that initial phage density = (plaque number/mL) × (dilution factor)

Answer 1

1st dilution: 0.1 mL of undiluted sample + 9.9 mL medium; Dilution factor = 1/100

2nd dilution: 0.1 mL of 1st dilution + 9.9 mL medium; Dilution factor = 1/100

3rd dilution: 0.1 mL of 2nd dilution + 9.9 mL medium; Dilution factor = 1/100

4th dilution: 0.1 mL of 3rd dilution + 9.9 mL medium; Dilution factor = 1/100

5th dilution: 0.1 mL of 4th dilution + 9.9 mL medium; Dilution factor = 1/100

Total dilution factor = (1/100) X (1/100) X (1/100) X (1/100) X (1/100)

= 1/10¹⁰

= 10^-11

0.1 mL = 30 plaques

1 mL = 300 plaques

Initial phage density = 300 X 1/10^-11

= 3 X 10¹³