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Questions and Answers
What is the primary goal of performing serial dilutions and plate counts?
Which of the following media types is used to assess genetic diversity in microbes?
Which of the following best describes how UV light damages bacterial DNA?
Why does UV light kill endospores at a lower rate than vegetative cells?
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A mutant bacterial strain may lose its ability to synthesize a particular amino acid due to disruption in the ____ pathway.
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What process allows one bacterial cell to form one colony on an agar plate?
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What characteristic is true of vegetative cells compared to endospores?
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What is a common consequence of spontaneous mutations in microbes?
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Study Notes
Serial Dilutions & Plate Count Lab
- Goal of serial dilutions and plate counts: Determine the concentration of viable bacterial cells in a sample.
- Principle of plate counting: One bacterial cell can form one colony on an agar plate.
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Creating serial dilutions:
- 10⁻¹ dilution: Transfer 1 mL of bacterial broth culture into 9 mL of sterile saline.
- 10⁻² dilution: Transfer 1 mL of the 10⁻¹ dilution into 9 mL of sterile saline.
- Dilution factor calculation: Transferring 1 mL from a 10⁻⁶ dilution into 9 mL of sterile saline results in a final dilution factor of 10⁻⁷.
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Calculating original bacterial concentration:
- Multiply the number of colonies by the dilution factor and the reciprocal of the volume plated.
- Example: 167 colonies x 10³ (10⁻³ dilution) x 10 (0.1 mL plated) = 1.67 x 10⁶ viable cells/mL
Genetic Diversity Lab
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Spontaneous Mutations:
- Random changes in DNA sequence occurring during DNA replication.
- May lead to altered phenotypes (e.g., antibiotic resistance, loss of enzyme function).
- Media for assessing genetic diversity: YNB agar (Yeast Nitrogen Base agar).
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Wild-type vs. Mutant Strains:
- Wild type: original, natural strain with no mutations.
- Mutant: strain with a specific genetic change that differentiates it from the wild type.
- Loss of amino acid synthesis: A mutant bacterial strain may lose its ability to synthesize a particular amino acid due to a disruption in its biosynthetic pathway.
UV Light Lab
- UV light damages bacterial DNA by: forming pyrimidine dimers (covalent bonds between adjacent thymine or cytosine bases).
- Pyrimidine dimers: Thymine dimers are the most common. They distort the DNA double helix and interfere with base pairing during DNA replication.
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Endospore UV resistance: Endospores are resistant to UV light because:
- They are metabolically inactive.
- They have DNA repair mechanisms.
- The spore coat provides protection.
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Description
Explore the principles of serial dilutions and plate counts in determining viable bacterial cell concentration. Learn how to perform dilutions, calculate dilution factors, and estimate original bacterial concentration through colony counting. This lab focuses on practical applications in microbiology.
