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 and 10-4

4. Place 1cc of the soil into the tube labeled 10

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 into 10-2

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 into 10-4

 

13. Plate 100µL of the corresponding solutions onto the petri plates labeled 10-2 , 10-3, and 10-4 using a diffenent sterile pipette each time.

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 # at which these colonies were found½

 

17. For safetly precautions, wash hands and rub alcohol onto workspace

 

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