Q: After plating 0.1 ml from tube C (from the previous question) and incubating, you count 68 colonies. What is the number of cells per ml in tube C?

$$68 \times 10 = 680$$ cells/ml

Q: Using the previous results, what is the number of cells per ml in the original fecal sample?

$$680 \times 1,000,000 = 6.8 \times 10^8$ cells/ml

Q: A urine sample is diluted by transferring 1 ml into 9 ml of media (tube X), then 1 ml from X into 9 ml of media (tube Y), and finally 0.1 ml from Y is plated. If 150 colonies are counted, what is the number of cells per ml in the original urine sample?

$$150 \times 100 = 15,000$$ cells/ml

Question 1

Which of the following best describes the difference between direct and indirect bacterial counting methods?

Accepted Answer

Direct methods involve counting cells visually or microscopically, while indirect methods estimate cell numbers based on measurements such as turbidity.

Question 2

A culture tube has become increasingly turbid over time. Which of the following changes would you expect to observe as turbidity increases?

Accepted Answer

Optical density (OD) increases; cell density increases; percent transmittance decreases.

Question 3

Which of the following is an example of a viable, direct counting method for bacteria?

Accepted Answer

Heterotrophic plate count

Question 4

Why is it important to plate each dilution three times when performing the heterotrophic plate count method?

Accepted Answer

To ensure statistical reliability and account for random variation in colony formation

Question 5

You are using a spectrophotometer to estimate bacterial cell numbers. Why must you agitate the tubes before inserting them into the spectrophotometer?

Accepted Answer

To resuspend any settled cells and ensure an accurate measurement of turbidity

Question 6

A student measures a culture with a spectrophotometer and records a percent transmittance (%T) of 20%. What is the optical density (OD) of the sample? Use the formula $$OD = -\log_{10}(	ext{T})$$, where $$	ext{T} is $$the decimal form of %T.

Accepted Answer

$$OD = -\log_{10}(0.2) = 0.6990$$

Question 7

Why should you not count plates with fewer than 30 colonies or more than 300 colonies when determining viable counts?

Accepted Answer

Plates with fewer than 30 colonies are statistically unreliable, and plates with more than 300 colonies may have overlapping colonies, making counts inaccurate.

Question 8

You perform a serial dilution by transferring 1 ml of a sample into 99 ml of media three times in succession (tubes A, B, and C). What is the total dilution factor in tube C?

Accepted Answer

$$1/1,000,000$$

Question 9

After plating 0.1 ml from tube C (from the previous question) and incubating, you count 68 colonies. What is the number of cells per ml in tube C?

Accepted Answer

$$68 	imes 10 = 680$$ cells/ml

Question 10

Using the previous results, what is the number of cells per ml in the original fecal sample?

Accepted Answer

$$680 	imes 1,000,000 = 6.8 	imes 10^8$ cells/ml

Question 11

A urine sample is diluted by transferring 1 ml into 9 ml of media (tube X), then 1 ml from X into 9 ml of media (tube Y), and finally 0.1 ml from Y is plated. If 150 colonies are counted, what is the number of cells per ml in the original urine sample?

Accepted Answer

$$150 	imes 100 = 15,000$$ cells/ml

Question 12

Which of the following best describes the difference between serial dilution and total dilution?

Accepted Answer

Serial dilution refers to the stepwise dilution process, while total dilution is the overall dilution factor after all steps.

Question 13

When is it more appropriate to use the spectrophotometry method rather than the heterotrophic plate count method for bacterial enumeration?

Accepted Answer

When you need a rapid estimate of total cell density, regardless of cell viability.

Question 14

Which of the following statements about percent transmittance (%T) and optical density (OD) is correct?

Accepted Answer

As %T decreases, OD increases.

Question 15

A student forgets to clean the cuvette before measuring OD in a spectrophotometer. What is the most likely consequence?

Accepted Answer

The OD reading will be artificially high due to light scattering by the dirty cuvette.

Question 16

Which of the following is NOT a limitation of using spectrophotometry for bacterial enumeration?

Accepted Answer

It can be used to identify bacterial species based on colony morphology.

Question 17

A researcher wants to determine the total number of bacteria (living and dead) in a sample. Which method would be most appropriate?

Accepted Answer

Spectrophotometry

Question 18

If a sample has an OD of 0.5 at 600 nm, and the standard curve shows that $$OD = 0.5$$ corresponds to $$5 	imes 10^8$ cells/ml, what is the estimated cell density?

Accepted Answer

$$5 	imes 10^8$ cells/ml

Question 19

Which of the following best explains why the heterotrophic plate count method is considered a viable count?

Accepted Answer

It only counts cells that are able to form colonies under the given conditions.

Microbiology Lab 5: Bacterial Enumeration Study Guide

Guida di studio - Domande di pratica

Study guide - Flashcard