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Calculating colony forming units?

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    • Thread Starter

    Hope this is in the right place, if not sorry!
    I'm a bit stuck on what i'm supposed to be doing in this calculation.. If anyone could give me some pointers i'd really appreciate it

    "Soil suspension has been prepared as follows: 10g soil was made up to 15ml with peptone solution. You are provided with a 10^-4 dilution of the original suspension. This was prepared in two steps; 0.1ml of suspension was added to 9.9ml peptone to give 10^-2 dilution. This was repeated to give the 10^-4 dilution.
    Calculate the number of colony forming units (CFUs) per gram of soil in your sample, take careful account of the dilution used for counting in your calculation"

    I prepared 10^-5 10^-6 and 10^-7 dilutions by removing 0.5ml each time to the new dilution.
    I plated 1ml samples of the four dilutions. Then after incubating I counted 300 colonies in the 10^-5 dilution..

    Now i'm stuck on what to do to calculate per ml or g .. I know to multiple the number of colonies by the dilution factor so 300 * 10^5 .. but do I need to do something else to get the answer?

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Updated: October 26, 2011
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