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Does this serial dilution question make sense?

  1. Nov 11, 2009 #1
    "You have a sample with a concentration of 1x106 bacteria/ml and you wish to know how many of these bacteria are viable. You dilute the sample down to a single bacterium/ml and plate 100 microliters of this diluted sample onto an agar dish. After incubating the dish for two weeks you count 122 colonies. What percentage of the bacteria in the initial sample were viable?"

    The number at the beginning is supposed to be the total number of bacteria in the sample, including dead and damaged cells (i.e. the number of bacteria you would get from physically counting the sample). But can you talk about a "total" number of bacteria in a sample like this, or does the number of bacteria we assign to samples always refer automatically to the number of viable bacteria?

    I'm just trying to figure out the standards on these kind of questions...

  2. jcsd
  3. Nov 12, 2009 #2

    Andy Resnick

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    Maybe I am misreading the question, but it appears very odd. The way I figure, you are plating (on average) 0.1 of a single bacterium, so your sample size is too small to make any statistical generalization.
  4. Nov 12, 2009 #3
    Must be one of those trick questions where the answer is contamination. :tongue:

    ...probably a typo?
  5. Nov 12, 2009 #4


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    I would assume so also. Otherwise I'd agree that you're measuring contamination, not actual bacteria in your sample. That, or it was a really poorly mixed dilution. :uhh:
  6. Nov 15, 2009 #5


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    How can you get 122 colonies out of 0.1 bacterium? As mentioned above, there is something wrong with the numbers in the initial question.
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