MathBench > Microbiology

Viable Plate Count, or...
How to count to a million

Design a dilution scheme!

So let’s put some of these skills together to design dilution schemes. To do this, you need to figure out the overall dilution you’re aiming for. In this case, just take the concentration you want to achieve, and divide it by the concentration you currently have. For example, if you want coffee with 5 caffeine molecules per cup, but you have coffee with 5000 molecules per cup, you have to prepare a dilution of 5/5000 or 1/1000.

The trickier part is figuring out a series of dilutions that will achieve this overall dilution. There is usually more than one way to achieve the same total dilution, and in deciding the best one to choose, you should avoid using volumes that are really large (this is wasteful and mixing may be a problem) or very small (sample taken may not be representative of the original sample). As any measurement or mixing errors are compounded when preparing a series of dilutions (or serial dilutions) it is generally advisable to minimize the number of steps.

In the examples in the applet below, the numbers always come out evenly if you choose dilutions of 1/2, 1/5 or 1/10, or their multiples. Use the scratchpad to check your plans, and if you get completely lost, click the ???! button for a workable dilution scheme.

You might not be able to do this in your sleep, but at least work at it until it makes sense…

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Design some dilutions...

You have a stock with an estimated 40,000 CFUs per mL, and you need to dilute to an estimated 500 CFUs per mL.

What overall dilution factor do you need?

How could you achieve this total dilution in three steps?
  • 1 mL --> mL water, then
  • 1 mL --> mL water, then
  • 1 mL --> mL water.
   
scratchpad
(test your dilution factors here):
* * =