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Questions and Answers
What is the primary purpose of the standard plate count method?
What is the primary purpose of the standard plate count method?
- To test the effectiveness of disinfectants
- To identify different species of microorganisms
- To measure the pH of a sample
- To estimate the population density in a liquid sample (correct)
In the serial dilution process, what is added to the original sample to create dilutions?
In the serial dilution process, what is added to the original sample to create dilutions?
- Oil
- Water, saline, or buffer (correct)
- Alcohol
- Nutrients
What does the term 'serial dilution' refer to?
What does the term 'serial dilution' refer to?
- A testing method for foodborne pathogens
- A single dilution made with multiple tubes
- A method to amplify the concentration of cells
- A series of controlled transfers down a line of dilution blanks (correct)
Which of the following best describes the expected outcome of a proper serial dilution?
Which of the following best describes the expected outcome of a proper serial dilution?
How is the inoculum transferred to a plate in the dilution process?
How is the inoculum transferred to a plate in the dilution process?
What does a 1/100 dilution express in scientific notation?
What does a 1/100 dilution express in scientific notation?
What is the acceptable range for colonies on a countable plate?
What is the acceptable range for colonies on a countable plate?
If 1 ml of a 10-1 dilution is transferred to 9 ml of diluent, what will be the resulting dilution?
If 1 ml of a 10-1 dilution is transferred to 9 ml of diluent, what will be the resulting dilution?
What does CFU stand for in the context of counting colonies?
What does CFU stand for in the context of counting colonies?
Which formula is used to calculate the original cell density (OCD)?
Which formula is used to calculate the original cell density (OCD)?
What does CFU stand for in microbiological calculations?
What does CFU stand for in microbiological calculations?
What is the maximum number of colonies allowed on a countable plate?
What is the maximum number of colonies allowed on a countable plate?
What does TFTC stand for in the context of plate counts?
What does TFTC stand for in the context of plate counts?
In a dilution series of three 10-1 dilutions, what is the final compounded dilution?
In a dilution series of three 10-1 dilutions, what is the final compounded dilution?
Which factor is included in the formula for calculating cell density expressed in CFU/ml?
Which factor is included in the formula for calculating cell density expressed in CFU/ml?
Why are CFUs preferred over using the total number of bacteria in microbiological studies?
Why are CFUs preferred over using the total number of bacteria in microbiological studies?
What is the effect of using unconventional volumes in dilution calculations?
What is the effect of using unconventional volumes in dilution calculations?
What is a primary consideration when determining if a plate is countable?
What is a primary consideration when determining if a plate is countable?
How is the final compounded dilution calculated for multiple simple dilutions?
How is the final compounded dilution calculated for multiple simple dilutions?
What indicates that the calculated cell density is accurate?
What indicates that the calculated cell density is accurate?
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Study Notes
Standard Plate Count (Viable Count)
- Standard plate count is a method to estimate microbial population density in liquid samples by counting colonies from diluted samples.
- Serial dilution involves incremental transfers through dilution blanks (usually sterile water, saline, or buffer) to create a range of sample concentrations.
- Each dilution aims to reduce the concentration of cells, producing a countable range of colonies on agar plates.
Procedure for Serial Dilution
- Utilize dilution tubes filled with known diluent volumes; for instance, 9.9 ml and 9.0 ml for different dilutions.
- Dilutions are expressed in scientific notation (e.g., 10^-1 for a 1/10 dilution) to denote concentration levels.
- A countable plate should contain between 30 to 300 colonies, with values below 30 deemed statistically unreliable.
Calculating Original Cell Density (OCD)
- Formula: OCD = (CFU × D) / V
- CFU = Colony Forming Units (number of colonies counted)
- D = Dilution factor from the tube
- V = Volume plated
- A notable consideration is that the dilution volume must be accounted for when determining original concentrations.
Countable Plate Standards
- Plates with fewer than 30 colonies are considered "too few to count" (TFTC).
- Plates exceeding 300 colonies likely exceed individual counting capabilities and are thus unmanageable.
Application in Microbial Analysis
- Viable count technique estimates the density of living cells in samples, aiding in microbiological analysis.
- Common practice includes plating dilutions onto agar using the spread plate technique to ensure even colony distribution.
Examples and Practical Applications
- Example calculations for different dilutions and colony counts help inform the standard plate count method.
- Conducting a dilution series and determining cell density of broth cultures, such as Escherichia coli, forms part of practical applications.
Additional Considerations in Dilution
- Understanding intended vs. actual dilution is crucial in experimental procedures, ensuring accuracy in microbial density assessments.
- Knowledge of assembly and handling techniques, such as sterilizing tools and transferring precise volumes, is critical for reliability in results.
Summary of Dilution Examples
- Various calculations can interpret when specific volumes are added to diluents incorrectly, highlighting the importance of precision in laboratory work.
- Keeping track of adjustments in volume and type will improve the accuracy of density calculations and ultimate microbial assessments.
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