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15.2: Serial Dilution

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    1. You have a urine sample from a patient that you suspect has a urinary tract infection. You make ten-fold dilutions of this sample as shown below, and then plate 0.5 ml (500 µl) of the last dilution on a TSA plate. There are 45 colonies on the plate. How many CFUs/ml were in the original urine sample?

    Screenshot (175).png

    2. You have received a sample from a sewage treatment plant, and have been asked to determine how many CFUs/ml are in this sample. You want to make a 1/100,000 fold dilution, but the smallest volume you can measure is 1.0 ml, and the tubes available to you only hold 10 ml. Explain/draw how you would do this!

    3. You have a bacterial culture that you know has 650,000 bacteria/ml. You do serial dilutions to achieve a 1/10,000 dilution, and then plate 0.1 ml of each of these dilutions. How many colonies will you see at each dilution?

    4. You do a series of dilutions as shown below, and you plate 1.0 ml of each dilution. Given the information below, fill in the number of colonies you would expect on each of the plates.

    Screenshot (176).png

    5. You do serial dilutions on a water sample, and plate the dilutions on TSA plates. You count the colonies on each of the plates as follows: (Note: TMTC = too many to count)

    Dilution Number of CFUs
    10-2 TMTC
    10-3 TMTC
    10-4 352
    10-5 45
    10-6 6
    10-7 0

    Based on these results, what is your estimate for the total number of CFUs/ml in the original sample?

    15.2: Serial Dilution is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Joan Petersen & Susan McLaughlin.

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