Serial Dilutions

Serial dilution is a technique used in many labs to create a gradient of reduced concentration of something. It could be reduced bacteria, antibiotics, food dye, or anything else you may want to reduce in a stepwise manner. Dilutions are often done in factors of 10⁻¹. You will need pipettes, a diluent, test tubes, and your sample.

General Procedure:

• Label each test tube in your dilution sequence (10⁻¹, 10⁻², 10⁻³, etc.)

• Add an equal amount of diluent to each tube

• Add a measured amount of your sample to the first tube and mix well

• Transfer a measured portion of the first dilution to the second dilution

• Transfer a measured portion of your second dilution to your third dilution

• repeat for all dilutions

From the general procedure, you can adapt it to fit whatever your needs may be. An example of this would be the serial dilutions found in my antibiotic discovery lab described below.

To perform a serial dilution, we prepared our materials by labeling our 5 sterile tubes with dilution factors, and added 900uL of sterile water to each tube. We then made our stock suspension by weighing out approximately 1g of soil from our soil sample and added it to a conical tube with 15mL of water. From this stock tube, we took 100uL and added it to the first 10⁻¹ tube. Then, we took 100uL from the 10⁻¹ tube and added it to the 10⁻². We repeated this process sequentially for each new tube to create the rest of the dilutions. Each dilution was then given its own corresponding plate, and 100 μL of each dilution was added and spread around each plate.