Essential Genetics 6th Edition by Daniel Hartl

Exercise 15

Numbers of phage or bacteria in a suspension are usually so large that single colonies or plaques would be impossible to observe without suitable dilution. The usual dilutions are 100- fold, in which 0.1 milliliter (ml) of the suspension is mixed with 9.9 ml of dilution buffer, or 10-fold, in which 1 ml of suspension is mixed with 9 ml of dilution buffer. Usually serial dilutions are necessary, in which the suspension is diluted once into dilution buffer, the resulting suspension mixed thoroughly and diluted a second time, the resulting suspension mixed thoroughly and diluted a third time, and so forth. The serial dilution factors multiply, so, for example, three 100-fold and two 10-fold dilutions yield an overall dilution factor of 100 × 100×100 × 10 × 10 =
. A volume of one ml of a
dilution is, therefore, the equivalent of a volume of
dilution of a suspension of bacteriophage T4 onto a lawn of E. coli and observes 16 plaques. What is the estimated number of T4 bacteriophage per milliliter in the original (undiluted) suspension?