Answers for Serial Dilution Problem # 1

1. Which is the countable plate? "C" #CFU = 200
2. What is the SDF? 1/2
3. Calculate the ITDF:
   1. 1 ml / 1 ml + 9 ml = 1/10
   2. 1 ml / 1 ml + 9 ml = 1/10
   3. 1 ml / 1 ml + 5 ml = 1/6
   4. 1 ml / 1 ml + 5 ml = 1/6
4. Calculate the TSDF for the countable plate.
   • 1/10 x 1/10 x 1/6 = 1/600
   • Note: ITDF for tube IV is not included; this is because it has no impact on the countable plate, "C". It only effects "D".
5. Calculate the PDF for the countable plate.
   • 0.3 ml/1 ml = 0.3 ml/1 ml x 10/10 = 3/10
   • Note: multiplying by 10/10 is just done for ease of calculations.
6. Calculate the FDF for the countable plate.
   • FDF = SDF x TSDF x PDF
   • FDF = 1/2 x 1/600 x 3/10 = 3/12,000 = 1/4,000
7. Calculate the CFU/ml for the countable plate.
   • CFU/ml = #CFU x 1/FDF
   • CFU/ml = 200 x 1 / 1/4,000
   • = 200 x 4,000
   • = 800,000 CFU/ml
   • = 8 x 10

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