Serial Dilutions
1. Use a pipette to add 10mL of distilled water to a 15 mL test tube. Label the sample number and label it 100
2. Use the same pipette to add
9mL of distilled water to another test tube. Label this tube 10-1
3. Repeat step 2 three times
more and label these tubes, 10-2 10-3
4. Place 1cc of the soil into the tube labeled 100
5. Cap and shake
6. Using a new pipette move 1mL
of the soil/water mixture into the tube labeled 10-1
7. Cap and shake
8. Using the pipette in step 5, move 1mL of the solution
from10-1
9. Cap and shake
10. Using the same pipette, move 1mL of the solution in
10-2 into 10-3
11. Cap and shake
12. Using the same pipette, move 1mL of the solution in
10-3
13. Plate 100µL of the corresponding solutions onto the
petri plates labeled 10-2 , 10-3
14. Allow to grow for 48 hours
15. Starting with the smallest dilution, see if there are five colonies of bacteria, if so count and record. If not, move up one dilution and so forth and record this.
16. Estimate the number of bacteria in the 1cc of soil by using the following formula
# of Microbes in 1cc of soil = # of Colonies on
sheet x 102 x 10
½dilution
#
17. For safetly precautions, wash hands and rub alcohol onto workspace