2.55: Exercise 15.1

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Page ID: 94510

Rosanna Hartline
Lemoore College

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Answer

211,000,000,000 cells/mL in original sample or 2.11 x 10¹¹ cells/mL in original sample

Solution

# of colonies counted = 211
amount of diluted sample added to the petri plate in mL = 100 μL = 0.1 mL
dilution of the petri plate counted = 10^-8

Step 1: Determine the concentration of cells in the diluted sample:

(# of colonies counted on the petri plate) ÷ (amount of diluted sample added to the petri plate in mL) = CFU in diluted sample (cells/mL)

(211 colonies) ÷ (0.1 mL diluted sample added to petri plate) = 2,110 cells/mL in the diluted sample

Step 2: Determine the concentration of cells in the original sample:

(CFU in diluted sample) ÷ (dilution of the petri plate counted) = CFU in original sample (cells/mL)

(2,110 cells/mL in diluted sample) ÷ (10^-8 dilution of the petri plate counted) = 211,000,000,000 cells/mL in original sample = 2.11 x 10¹¹ cells/mL in original sample

CFU = 211,000,000,000 cells/mL in original sample or 2.11 x 10¹¹ cells/mL in original sample

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