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Homework Help: Calculating the Original Concentration of Bacteria

  1. Aug 22, 2008 #1
    I am not really sure how to find the original concentration of bacteria for my experiment.

    This is what i did:
    I made serial dilutions of 10^-2, 10^-4,10^-6, 10^-8 with stock bacteria.
    From 10^-4 dilution, I took out 2ml and inoculated into 20g potatoes + 180g buffer. Potatoes + buffer + inoculum were homogenized. Then I took 0.1 ml from the homogenate and plate onto agar.

    If i want to find the original concentration in my stock bacteria, is this what I do:

    plate count = 30 colonies

    (30 x 10^4) / (10)(2)
    = 1.45x10^6 cfu/ml

    can someone check if my calculations are correct? Thanks!
  2. jcsd
  3. Aug 22, 2008 #2
    the equation should actually read:

    (30 x 10^4) / (0.1)(2)
    = 1.45 x 10^6 cfu/ml
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