Question

You have a cell suspension that has 10⁷ cells/mL and you serially dilute the culture to obtain the following dilutions: 10^-1, 10^-2, 10^-3, 10^-4, 10^-5, 10^-6. Which of the dilutions will yield a “countable” number of colonies (30-300 per plate) when you plate 0.1 ml of each dilution? Show your work.

Answer 1

a countable number of colonies is 30 to 300.

there are 10^7 cells/mL

so let`s take a countable number of colonies be 100 ( CFU/mL= number of colonies/ ( volume plated in mL dilution), here all the dilutions are some power to 10, 10-1, 10-2, 10-3, 10-4, 10-5, 10-6, so theoretically number of colonies in the countable range is 10^7/10^x, the value of countable number of colonies in the range of 30-300 is 100)

CFU/mL= number of colonies/ ( volume plated in mL dilution)

10^7 cells/mL= 100/( 0.1dilution)

dilution= 100/(0.110^7)

= 10^-4

so the dilution 10^-4 will give a countable range of colonies)