How to Work Microbiology Dilution Problems

••• Bacteria Colonies image by ggw from

Dilutions are useful in science when making solutions or growing an acceptable number of bacterial colonies to count. There are three formulas used to work microbiology dilution problems: finding individual dilutions, finding serial dilutions, and finding the number of organisms in the original sample.

    Finding the individual dilution for each tube is essential for dilution problems.
    ••• test tubes image by Paul Hill from

    To find a dilution of a single tube, use the formula: sample/(diluent + sample). The sample is the amount you are transferring into the tube, and the diluent is the liquid already in the tube. When you transfer 1 ml into 9 mls, the formula would be: 1/(1+9) = 1/10. This could also be written as 1:10.

    For serial dilutions, multiply the individual dilutions together to get the final dilution.
    ••• blank mixing tubes image by Allyson Ricketts from

    After you have calculated the individual dilutions for each tube, multiply the dilutions when using serial dilutions. Serial dilutions are the culmination of a number of diluted tubes used in order to get smaller dilutions. When a sample diluted 1/100 is added to a sample diluted 1/10, the final dilution would be: (1/100) x (1/10) = 1/1000.

    The number of organisms in the original sample will always be greater than in any of the diluted tubes.
    ••• bacteria, image by chrisharvey from

    Use this equation to determine the number of organisms in the original sample once you have found the dilution: number of colonies growing on plate x (1/volume used to put bacteria on plate) x (1/dilution). When a student uses 1ml on a plate from a tube diluted 1/100, and the plate grew 230 colonies, the formula would be: 230 x (1/1ml) x (1/(1/100)) = 23000 or 2.3 x 10^4.


  • Microbiology Lab Manual TCCD South Campus; Chess, Kleyn, Bicknell, Brown, and Harley; 2009

About the Author

Marcia Spear holds a Master of Science in biomedical sciences with an emphasis in Immunology and microbiology. Spear has taught science classes for Weatherford College, Tarrant County College, Dallas County College and HIll College since 2010. Her classes include Biology Majors, Biology Non-Majors, Microbiology, and Anatomy and Physiology.

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