뒤로Microbiology Lab 5: Bacterial Enumeration Study Guide
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Q1. What is the difference between direct and indirect counting methods for bacteria?
Background
Topic: Bacterial Enumeration Methods
This question tests your understanding of the two main approaches to quantifying bacteria in a sample: direct and indirect methods.
Key Terms:
Direct counting: Methods that involve directly observing and counting bacterial cells.
Indirect counting: Methods that estimate cell numbers based on measurements related to bacterial growth or activity.
Step-by-Step Guidance
Recall what is meant by 'direct' in the context of microbiology—think about methods where you actually see or count the cells themselves.
Consider what 'indirect' means—these methods do not count cells directly but use other indicators (like turbidity or metabolic activity) to estimate cell numbers.
Think of examples of each type (e.g., microscopic counts for direct, spectrophotometry for indirect).
Reflect on the advantages and limitations of each method.
Try explaining the differences in your own words before checking the answer!
Final Answer:
Direct counting methods involve physically counting individual bacterial cells, such as using a microscope with a counting chamber or performing plate counts. Indirect counting methods estimate the number of bacteria based on measurements like turbidity (optical density), metabolic activity, or other indirect indicators. Direct methods provide actual cell counts, while indirect methods infer cell numbers from related measurements.
Q2. What is the difference between viable and total counting methods for bacteria?
Background
Topic: Viable vs. Total Cell Counts
This question focuses on distinguishing between methods that count only living (viable) cells and those that count all cells, living or dead.
Key Terms:
Viable count: Counts only living cells capable of forming colonies.
Total count: Counts all cells, regardless of viability.
Step-by-Step Guidance
Recall what 'viable' means in microbiology—cells that are alive and able to reproduce.
Think about which methods can distinguish between live and dead cells (e.g., plate counts for viable, microscopic counts for total).
Consider why it might be important to know the difference in certain applications (e.g., food safety, clinical diagnostics).
Try to define each method before revealing the answer!
Final Answer:
Viable counting methods only count living cells that can grow and form colonies, such as the plate count method. Total counting methods count all cells present, both living and dead, such as direct microscopic counts or spectrophotometric measurements. Viable counts are important for determining the number of cells capable of causing infection or spoilage.
Q3. What are examples of direct and indirect counting methods and viable and total counting methods? Make sure you know the ones discussed in lab.
Background
Topic: Examples of Enumeration Methods
This question asks you to recall specific laboratory techniques and classify them as direct/indirect and viable/total.
Key Terms:
Direct methods: Microscopic count, plate count.
Indirect methods: Turbidity measurement, metabolic activity assays.
Viable methods: Plate count.
Total methods: Microscopic count, turbidity.
Step-by-Step Guidance
List the methods you learned in lab for counting bacteria.
Classify each method as direct or indirect based on whether you count cells directly or estimate their number.
Classify each method as viable or total based on whether it counts only living cells or all cells.
Make a table or chart to help organize your thoughts.
Try to fill in your own table before checking the answer!
Final Answer:
Direct & Viable: Plate count (heterotrophic plate count method). Direct & Total: Microscopic count using a counting chamber. Indirect & Total: Turbidity measurement (spectrophotometry). Indirect & Viable: Not commonly used, but metabolic activity assays can sometimes estimate viable cells indirectly.
Q4. Explain how the turbidity of a culture relates to its cell density, OD, and % transmittance.
Background
Topic: Spectrophotometry and Bacterial Growth
This question tests your understanding of how optical measurements relate to bacterial concentration in a liquid culture.
Key Terms and Formulas:
Turbidity: Cloudiness of a culture due to suspended cells.
OD (Optical Density): A measure of how much light is absorbed by the culture.
% Transmittance: The percentage of light that passes through the culture.
Relationship:
Step-by-Step Guidance
Recall that as cell density increases, the culture becomes more turbid (cloudy).
Understand that increased turbidity means more light is scattered or absorbed, so less light passes through (lower % transmittance).
Recognize that OD increases as turbidity increases, since OD measures how much light is absorbed.
Use the formula to relate OD and % transmittance.
Try to fill in the blanks in the example statements using this logic.
Try to complete the relationships before revealing the answer!
Final Answer:
As turbidity increases, OD increases, cell density increases, and % transmittance decreases. As cell density increases, OD increases and % transmittance decreases. This is because more cells scatter and absorb more light, reducing the amount that passes through the sample.
Q5. Distinguish between serial dilution and total dilution.
Background
Topic: Dilution Techniques in Microbiology
This question tests your understanding of how to prepare and calculate dilutions in the lab.
Key Terms:
Serial dilution: A stepwise dilution of a substance in solution.
Total dilution: The overall dilution factor from the original sample to the final dilution.
Step-by-Step Guidance
Recall that serial dilution involves making a series of smaller, sequential dilutions.
Understand that total dilution is the product of all individual dilution factors in the series.
Think about how you would calculate the total dilution if you performed three 1:10 serial dilutions.
Try to define each term and calculate a sample total dilution before checking the answer!
Final Answer:
Serial dilution refers to the process of diluting a sample in steps, each time by a known factor (e.g., 1:10). Total dilution is the cumulative dilution factor after all steps, calculated by multiplying the individual dilution factors together (e.g., three 1:10 dilutions = 1:1000 total dilution).
Q6. What does percent transmittance mean? What does OD mean?
Background
Topic: Spectrophotometry in Microbiology
This question asks you to define two key terms used in measuring bacterial growth using a spectrophotometer.
Key Terms and Formulas:
% Transmittance (%T): The percentage of light that passes through a sample.
OD (Optical Density): A logarithmic measure of how much light is absorbed by the sample.
Formula:
Step-by-Step Guidance
Recall that a spectrophotometer measures how much light passes through a sample (%T).
Understand that OD is calculated from %T and reflects the sample's turbidity.
Think about what a high %T or high OD means in terms of cell density.
Try to define each term in your own words before checking the answer!
Final Answer:
Percent transmittance (%T) is the percentage of light that passes through a sample compared to a blank. Optical density (OD) is a measure of how much light is absorbed by the sample, calculated as . Higher OD means more cells are present, making the sample more turbid.
Q7a. When doing the heterotrophic plate count method to determine viable cells in a culture: Why must you plate each dilution 3 times?
Background
Topic: Plate Count Technique and Replicates
This question tests your understanding of why replicates are important in microbiological quantification.
Key Terms:
Replicates: Multiple samples or plates prepared in the same way.
Step-by-Step Guidance
Recall that biological samples can have variability due to sampling and technique.
Think about how using replicates can help ensure accuracy and reliability of your results.
Consider what you would do if one plate had an outlier result compared to the others.
Try to explain the importance of replicates before checking the answer!
Final Answer:
Plating each dilution three times (in triplicate) helps account for variability and increases the reliability of your results. It allows you to identify outliers and calculate an average, making your cell count more accurate and reproducible.
Q7b. Why must you not count the plates that have over 300 colonies or under 30 colonies to determine viable counts?
Background
Topic: Plate Count Range and Accuracy
This question addresses the concept of countable plate ranges in microbiology.
Key Terms:
Countable range: The range of colony numbers considered statistically reliable (usually 30–300 colonies per plate).
Step-by-Step Guidance
Recall that too few colonies can lead to high statistical error, while too many can cause colonies to merge.
Think about how overcrowding or too few colonies can affect the accuracy of your count.
Consider why a standard range (30–300) is used in microbiology labs.
Try to explain the reasoning before checking the answer!
Final Answer:
Plates with fewer than 30 colonies are statistically unreliable due to sampling error, and plates with more than 300 colonies are too crowded, making it difficult to distinguish individual colonies. Only plates within the 30–300 range are considered accurate for viable counts.
Q8a. When doing the spectroscopy method to determine cell numbers: Why must you use clean cuvettes?
Background
Topic: Spectrophotometry Technique
This question tests your understanding of proper technique when using a spectrophotometer.
Key Terms:
Cuvette: A small, transparent container used in spectrophotometry.
Step-by-Step Guidance
Recall that the spectrophotometer measures how much light passes through the cuvette.
Think about what could happen if the cuvette is dirty or has fingerprints.
Consider how this could affect your OD readings.
Try to explain why cleanliness is important before checking the answer!
Final Answer:
Clean cuvettes are essential because any residue, fingerprints, or scratches can scatter or absorb light, leading to inaccurate OD readings. This ensures that only the sample affects the measurement.
Q8b. Why must you agitate the tubes before inserting them in the spectrophotometer?
Background
Topic: Sample Preparation for Spectrophotometry
This question focuses on why mixing the sample is important before measurement.
Key Terms:
Agitation: Mixing the sample to ensure even distribution of cells.
Step-by-Step Guidance
Recall that bacteria can settle at the bottom of the tube if left undisturbed.
Think about how uneven distribution could affect the accuracy of your OD measurement.
Consider why a homogeneous suspension is necessary for reliable results.
Try to explain the importance of agitation before checking the answer!
Final Answer:
Agitating the tubes ensures that the bacterial cells are evenly distributed throughout the sample, providing an accurate and representative OD measurement. Without mixing, the reading may underestimate cell density due to settling.
Q9. When is it appropriate to use the spectroscopy method to count bacteria and when is it appropriate to perform the heterotrophic plate count method?
Background
Topic: Choosing Enumeration Methods
This question asks you to consider the strengths and limitations of each method and when each is best used.
Key Terms:
Spectroscopy method: Measures total cell density (living and dead) via turbidity.
Heterotrophic plate count: Measures only viable (living) cells capable of forming colonies.
Step-by-Step Guidance
Recall that spectrophotometry is quick and useful for monitoring growth trends but does not distinguish live from dead cells.
Understand that plate counts are more time-consuming but provide information on viable cells.
Think about situations where you need to know total cell mass versus viable cell numbers.
Try to match each method to its best use before checking the answer!
Final Answer:
The spectroscopy method is appropriate for quickly estimating total cell density in a culture, especially when monitoring growth over time. The heterotrophic plate count method is used when you need to know the number of viable (living) cells, such as in clinical or food safety applications.
Q10. Serial Dilution Calculation Example: Fecal Sample
Background
Topic: Serial Dilution and Plate Count Calculations
This multi-part question tests your ability to calculate dilution factors and determine cell concentrations from a serial dilution and plate count experiment.
Key Terms and Formulas:
Serial dilution: Stepwise dilution of a sample.
Total dilution factor: Product of all individual dilution steps.
CFU (colony-forming units): Number of viable cells per unit volume.
Formula for CFU/ml:
Step-by-Step Guidance
For each dilution step, calculate the dilution factor (e.g., 1 ml into 99 ml = 1:100 dilution).
Multiply the dilution factors for each step to get the total dilution at each tube (A, B, C).
For the plate count, note the volume plated (0.1 ml) and the number of colonies counted (68).
Set up the formula for CFU/ml in tube C using the number of colonies, volume plated, and total dilution factor.
To find the number of cells per ml in the original fecal sample, use the total dilution from the original sample to the plate.
Try to calculate each step before revealing the answer!
Final Answer:
1) Serial and total dilutions: Tube A: 1:100 Tube B: 1:100 (from A), so total = 1:10,000 Tube C: 1:100 (from B), so total = 1:1,000,000
2) Number of cells per ml in tube C: CFU/ml = 68 colonies / (0.1 ml plated) = 680 CFU/ml in tube C.
3) Number of cells per ml in the original fecal sample: Total dilution from original to plate is 1:1,000,000, so CFU/ml in original = 680 CFU/ml × 1,000,000 = 6.8 × 108 CFU/ml.
We used the plate count and dilution factors to back-calculate the concentration in the original sample.
Q11. Urine Sample Dilution Calculation Example
Background
Topic: Serial Dilution and Plate Count Calculations
This question tests your ability to calculate the number of bacteria per ml in a urine sample using a dilution scheme and plate count.
Key Terms and Formulas:
Serial dilution: Stepwise dilution of a sample.
Total dilution factor: Product of all individual dilution steps.
CFU (colony-forming units): Number of viable cells per unit volume.
Formula for CFU/ml:
Step-by-Step Guidance
Calculate the dilution factor for each step (1 ml into 9 ml = 1:10 dilution).
Multiply the dilution factors for each step to get the total dilution from the original sample to the plate.
Note the volume plated (0.1 ml) and the number of colonies counted (150).
Set up the formula for CFU/ml in the original urine sample using the number of colonies, volume plated, and total dilution factor.
Plug in the values to solve for CFU/ml.
Try to set up the calculation before revealing the answer!
Final Answer:
Total dilution: 1:10 (X) × 1:10 (Y) = 1:100. Volume plated = 0.1 ml. CFU/ml = 150 / (0.1 × 1/100) = 150 / 0.001 = 1.5 × 105 CFU/ml in the original urine sample.
This calculation shows the concentration of E. coli in the patient's urine.
