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Home Q & A Board Laboratory Help Introductory Courses
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Leen Leen
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10 years ago
Manny tells you that he diluted his bacteria. His dilution series was done using eppendorf tubes with a maximum of 1ml in them. The concentration of bacteria before starting was 1000/ml. The number of bacteria on his first plate was 33. This plate had 0.1ml of material from the first tube in the dilution series. What factor dilution did Manny use?
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#1
10 years ago
Starting concentration = 1000 bacteria/mL.

Let the first dilution factor = F, so that the initial dilution was 1/F,
that is, the concentration in the first tube was 1000/F bacteria/mL.

0.1 mL of this was plated, which equals 1/10 of 1000/F = 100/F bacteria.

But this is equal to 33 colonies (equivalent to 33 viable bacteria).

Therefore, 100/F = 33, so F = 100/33, which is close enough to F = 3.

Now if we assume that F = 3, then 100/F = 33.3 (which is close enough to 33).
So it looks like the dilution factor was 3.
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