Answers for Serial Dilution Problem # 5

1. Which is the countable plate? "D" #CFU 40
2. What is the SDF? 1/200
3. Calculate the ITDF:
   1. 0.1 ml / 0.1 ml + 0.9 ml = 0.1 ml / 1.0 ml = 1/10
   2. See Above = 1/10
   3. 0.01 ml / 0.01 ml + 0.99 ml = 0.01 ml / 1.0 ml = 1/100
   4. See Above = 1/100
4. Calculate the TSDF for the countable plate.
   • 1/10 x 1/10 x 1/100 x 1/100 = 1,000,000
5. Calculate the PDF for the countable plate.
   • 0.1 ml/1 ml = 1/10
6. Calculate the FDF for the countable plate.
   • FDF = SDF x TSDF x PDF
   • FDF = 1/200 x 1/1,000,000 x 1/10 = 1/2,000,000,000
7. Calculate the CFU/ml for the countable plate.
   • CFU/ml = #CFU x 1/FDF
   • CFU/ml = 40 x 1 / 1/2,000,000,000
   • = 40 x 2,000,000,000
   • = 80,000,000,000 CFU/ml
   • = 8.0 x 10CFU/ml

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