Microbiology - 002 - Serial Dilutions

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Sometimes a solution is too concentrated to work with. A serial dilution is a series of successive measured dilutions. As illustrated here with a serial dilution of dye, each dilution contains less of the original sample. This same technique can be used to dilute a liquid culture of bacteria. 

This is a serological pipette. It is used to transport a specific volume of liquid. Printed marks along the side measure the volume of liquid in the pipette. As long as it remains sealed, the pipette is sterile. 

Use the blunt end of the pipette to pierce its wrapper, but keep the pipette in the wrapper so it remains sterile. Insert the blunt end into a pipette pump, which is a device used to draw liquid into the pipette using suction power. When you are ready to transfer liquid, grasp the pipette pump and remove the pipette from the wrapper. Place the tip of the pipette into the source liquid. Carefully move the wheel of the pipette pump and watch the liquid in the tube; stop when it reaches the mark for the desired volume. Move the pipette to the destination and dispense the liquid by pulling the trigger, moving the wheel, or pressing on the plunger of the pipette pump. 

Dispose of the pipette after transfer; they are single-use only. Do not return the pipette to its wrapper; place it in the designated pipette bins. The wrapper should be thrown away elsewhere. Never use a pipette more than once; if you need to transfer another volume of liquid, always use a fresh pipette. 

As you can see, the liquid does not disperse completely after transfer. The solution must be mixed by gently inverting the tube or by vortexing. Mixing the sample is extremely important. 

The transfer procedure can be repeated as many times as necessary to get the desired number of dilutions. With the proper technique, the dilutions will cover a range of concentrations in discrete, consistent, measurable steps.

Sometimes a solution of bacteria is too concentrated to work with. A serial dilution is a series of successive, measured dilutions in order to obtain a known, workable concentration of bacteria.

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