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 -- you don't want to be a lab grunt all your life, right?

So to do this, you need to figure out the overall dilution you're shooting for. In this case, just take the concentration you want to achieve, and divide it by the concentration you currently have. As an example, if you want coffee with 5 caffeine molecules per mL, but the coffee you have has 5000, you need a dilution of 5/5000, or 0.001.

The trickier part is figuring out a series of dilutions that will achieve your overall dilution. In these examples, the numbers will always come out evenly if you choose dilutions of 1/2, 1/5, 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...

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.
(test your dilution factors here):

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