How to Calculate Serial Dilutions in Microbiology

Serial dilutions are used in biological and chemical laboratories primarily for experimental purposes. These dilutions are often done in a logarithmic fashion or completed by factors of 10's. Most often bacteria, cells or solutes are diluted to act as controls in experiments. Dilutions serve as an accurate control when used in dose-response experiments since all samples originated from the same lot. In addition, dilutions might be necessary to reduce the number of particles in a sample for use in automated equipment. Serial dilutions are completed by taking a small volume from the original sample and placing it in a new vile and returning it to the original volume with the diluting solution. The general formula is: original concentration/dilution factor = new concentration. This example is a serial dilution of 10 mL of 2.5x10^7 cells/mL to 25 cells/mL by a dilution of 100.

Things You'll Need

• Protective eye goggles
• Gloves
• 10 mL of 2.5x10^7 cells/mL
• Three 100 mL test tubes
• Wax pencil
• 1 mL Pipette
• 100 mL Pipette

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Instructions

• 1

  Place protective goggles over eyes and put on gloves before handling substances.

• 2

  Label the three test tubes with the wax pencil: 2.5 x 10^5, 2.5 x 10^3, 25 cells/mL.

• 3

  Use the formula: original concentration of cells/dilution factor= new concentration to solve for the first dilution. The original concentration is 2.5 x 10^7 and there is a dilution factor of 100 for a final new concentration of 2.5 x 10^5 cells/mL.

• 4

  Dilute the original sample of 2.5 x 10^7 cells/mL by a factor of 100. Pipette 1 mL of the original solution into test tube labeled "2.5 x 10^5 cells/mL." Pipette an additional 99 mL of water into the test tube to complete the dilution of 2.5 x 10^5 cells/mL.

• 5

  Dilute the cells by another factor of 100. Pipette 1 mL from the 2.5 x 10^5 cells/mL test tube into the tube labeled "2.5 x 10^3 cells/mL." Pipette an additional 99 mL of water to the "2.5 x 10^3 cells/mL" tube to complete the dilution to 2.5 x 10^3 cells/mL.

• 6

  Dilute the cells by the final factor of 100. Pipette 1 mL from the 2.5 x 10^3 cells/mL test tube into the tube labeled "25 cells/mL." Pipette an additional 99 mL of water to the "25 cells/mL" to complete the final dilution to 25 cells/mL.