Serial Dilution Problem Help

Serial dilutions are used to calculate the concentration of microorganisms. As it would usually be impossible to actually count the number of microorganisms in a sample, the sample is diluted and plated to get a reasonable number of colonies to count. Since each colony on an agar plate theoretically grew from a single microorganism, the number of colonies or Colony Forming Units is representative of the number of viable microorganisms. Since the dilution factor is known, the number of microorganisms per ml in the original sample can be calculated.

A dilution problem such as the one shown above is relatively easy to solve if taken step by step. Follow the steps below.

  1. First determine which is the countable plate.
  2. Sample Dilution Factor (SDF)
  3. Individual Tube Dilution Factor (ITDF)
  4. Total Series Dilution Factor (TSDF)
  5. Plating Dilution Factor (PDF)
  6. Final Dilution Factor (FDF)
  7. Colony Forming Units/ml (CFU/ml) in original sample

Note: Thanks are given to Tim Allen for creating this help unit.

First Problem


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