Problem 2 answer

Let’s go through the 4 steps.

1.     Find the plate that resulted in between 30-300 colonies.  Which tube did this come from?

Tube C had 50 colonies

2.     You need to calculate the serial dilutions of all of the tubes up to and including your tube that resulted in the 30 – 300 colonies.

SD Tube A =   1/(1+9) = 1/10 = 10^-1

SD Tube B = 1/(1 + 99) = 1/100 = 10^-2

SD Tube C = 0.1/(0.1 + 9.9) = 0.1/10 = 1/100 = 10^-2

3.     You must determine the total dilution in your tube that resulted in the 30 – 300 colonies. To do this, multiply all of the serial dilutions up to this tube.

You must then change the negative sign in the exponent to a positive sign.

Total dilution = SD Tube A X SD Tube B X SD Tube C

                        = 10^-1 X 10^-2 X 10^-2 = 10^-5

Total dilution = 10⁵

4.     Multiply the number of colonies by the total dilution and by 10 if you plated 0.1 ml.

50 X 10⁵ X 10 = 500 X 10⁵ or 5 X 10⁷ cells/ml

The concentration of cells in your original culture was 5 X 10⁷ cells/ml.