You have a culture of yeast that is at a concentration of 6.74 x 10^6 cells/ml. You dilute the sample 1:100, and then 1:100 again, and finally you dilute the sample an additional 1:3. You add 0.1 ml of the final dilution to a spread plate.
1. Assuming that most of the cells in the original culture were living, how many CFUs do you expect to count on your spread plate the next day?

22 cells

Explanation:

The concentration of numbers of cells = 6.74 x 10⁶ cells/ml

Dilution stages includes =  1:100  ;     1:100    &        1:3

The consecutive dilution stages can be calculated as:

= (6.74 x 10⁶) × (1/100)(1/100)(1/3)

=  222.42 cells/ml

= 2.22 × 10² cells/ml

So after addition  0.1 ml of the final dilution to a spread plate, the number of CFUs ( Colony forming units) we expect to count will be:

= (2.22 × 10² cells/ml)(0.1 ml)

= 22.2 cells

≅ 22 cells.