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    Serial dilution problem

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    You have decided to determine how many microbes are living on the lettuce in the salad bar at your favorite restaurant. You place 1 gram of lettuce and 99 mls of water in a blender and blend the mixture. This is sample A. You then transfer 1 ml of this dilution into to another that contains 9 mls of water. This becomes sample B. You next transfer 1 ml of sample B into a separate container that contains 9 mls of water. This is sample C. Next you transfer 1 ml each from samples B & C onto separate nutrient rich agar plates, swirl, let harden and incubate at 37C. When you examine the plates after 48 hours you find 110 colonies growing on plate C. (For your information 1 gram 1 ml)

    -what was the bacterial count in SAMPLE A?
    -would you expect to find more colonies growing on the plate that was inoculated from sample B or sample C?
    -how many microbes were living on that 1 gram of lettuce.

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    Solution Preview

    To find the bacterial count in Sample A, we must work backwards.

    In 1 mL of Sample C, we counted 110 colonies. Each colony grew from a single bacterial cell. Therefore, there was 110 bacteria/mL in Sample C. But, we ...