Answers for Serial Dilution Problem # 3

1. Which is the countable plate? "B" # PFU = 100
2. What is the SDF? 1/1
3. Calculate the ITDF for each tube:
   1. 0.2 µl / 0.2 µl + 0.8 µl = 0.2 / 1.0 = 1/5
   2. see above = 1/5
   3. see above = 1/5
   4. see above = 1/5
4. Calculate the TSDF for the countable plate.
   • 1/5 x 1/5 = 1/25
5. Calculate the PDF for the countable plate.
   • 0.1 µl l /1,000 µl = 1/10,000
6. Calculate the FDF for the countable plate.
   • FDF = SDF x TSDF x PDF
   • FDF = 1/1 x 1/25 x 1/10,000 = 1/250,000
7. Calculate the PFU/ml for the countable plate.
   • PFU/ml = #PFU x 1/FDF
   • PFU/ml = 100 x 1 / 1/250,000
   • = 100 x 250,000
   • = 25,000,000 PFU/ml
   • = 2.5 x 10

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