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Measuring Growth by Plate Counts quiz

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  • What do plate counts specifically measure in a microbial culture?
    Plate counts measure the number of viable (living) cells in a culture.
  • Why might a scientist prefer plate counts over direct cell counting?
    Plate counts only count viable cells, while direct cell counting includes both living and dead cells.
  • What is the critical assumption made in the plate count method?
    The method assumes that each colony on a plate originates from a single viable cell.
  • What does CFU stand for, and what does it represent?
    CFU stands for Colony Forming Unit, representing a single viable cell capable of forming a colony.
  • Why are serial dilutions necessary in the plate count method?
    Serial dilutions are needed to obtain a countable number of colonies, typically between 33 and 100.
  • What is the typical range of colonies considered countable and reliable on a plate?
    A countable and reliable plate has between 33 and 100 colonies.
  • What is the first step in preparing for a plate count?
    Cells are first grown in a liquid culture before being transferred to solid growth media.
  • How is a 1:10 dilution performed during serial dilutions?
    One milliliter of the original culture is added to nine milliliters of broth, creating a 1:10 dilution.
  • What happens if you plate the original undiluted culture?
    There will be too many colonies to count accurately, making the results unreliable.
  • How can you calculate the number of viable cells in the original culture after counting colonies?
    By knowing the dilution factor and the number of colonies, you can reverse-calculate the number of viable cells in the original culture.
  • What does it mean if there are fewer than 33 colonies on a plate?
    The count is considered unreliable because the number of colonies is too low.
  • What does it mean if there are more than 100 colonies on a plate?
    The count is considered unreliable because the colonies are too numerous to count accurately.
  • Why is it important to know which dilution was used for counting colonies?
    Knowing the dilution allows accurate calculation of the original number of viable cells.
  • What is the main advantage of using plate counts for measuring microbial growth?
    Plate counts provide an accurate count of only the living, viable cells in a sample.
  • What is the relationship between the number of colonies and the number of viable cells plated?
    The number of colonies equals the number of viable cells (CFUs) that were plated, assuming each colony arises from one cell.