Practice your understanding of dilutions with the following problems:
1. You perform a serial dilution.
First dilution - pipette 1 ml of culture into 9 ml of water.
Second dilution - pipette 1 ml of first dilution into another 9 ml of water.
Third dilution - pipette 1ml of the second dilution into 99 ml of water.
Diagram this procedure. What is the total dilution?
2. You perform a serial dilution starting with a culture containing 1,000,000 organisms/ml.
First dilution - pipette 1 ml of culture into 99 ml of water.
Second dilution - pipette 1 ml of first dilution into another 99 ml of water.
Third dilution - pipette 1ml of the second dilution into 9 ml of water.
What is the total dilution at each step? What is the number of organisms/ml at each step?
3. In the previous question, you prepare pour plates from the second dilution by pipetting either 0.1 ml or 1 ml samples. How many organisms would you expect on these plates?
4. You have a culture containing 109 organisms/ml and wish to dilute it to 10 2 (100) organisms/ml. Design a serial dilution.
5. You have performed the following serial dilutions: 1:100, 1:100, 1:10. Diagram this dilution scheme and include the overall dilution in each tube.
From each dilution, you then prepare pour plates using either 0.1 ml or 1 ml samples. After incubation, you count the plates and get the following results:
What was the number of organisms/ml in the initial culture?
Final dilution | No of colonies 1 ml | No. of colonies 0.1 ml
1:100 | TNTC | TNTC
1:10,000 | TNTC | 63
1:100,000 | 59 | 7