Skip to main content
Biology LibreTexts

2.55: Exercise 15.1

  • Page ID
    94510
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    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


    This page titled 2.55: Exercise 15.1 is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.

    • Was this article helpful?