You prepare a pour plate using 1 ml of a 105 phage dilution

You prepare a pour plate using .1 ml of a 10-5 phage dilution and .5 mls of bacteria. You count 75 plaques. How many phage were in a ml of the undiluted phage stock?

Solution

Bacteriophage infects bcteria and kill them. This creates a circular and empty (devoid of bacteril cells) region around themselves. Each bacteriophage corresponds to one plaque in a cultured bacterial lawn.

In the question,

Number of plaques = 75

These plaques were obtained from a 10^-5 diluted stock. Hence back calclultion will yield the actual number of phages in 0.1 ml of taken stock.

= 75/10^-5 = 7.5x10⁶

This number was present in 0.1 ml stock. 1 ml of the stock will contain = 7.5x10⁶/0.1 = 7.5x10⁷phages

7.5x10⁷ phages were there in a ml of the undiluted phage stock