Q: A microbiologist counts 120 cells in a defined area of a Petroff-Hausser counting chamber, which holds $$1 \times 10$$^{-4}$$ ml. What is the concentration of cells per ml in the original sample?

$$1.2 \times 10$$^{6}$$ cells/ml$$\ (120\ cells/1 \times $$10^{-4}$$\ ml = 1.2 \times $$10^{6}$$\ cells/ml)$$

Question 1

Which instrument is shown in the first image, and what is its primary use in microbiology?

Accepted Answer

A Petroff-Hausser counting chamber, used for direct microscopic counts of microorganisms.

Question 2

A microbiologist counts 120 cells in a defined area of a Petroff-Hausser counting chamber, which holds $$1 	imes 10$$^{-4}$$ ml. What is the concentration of cells per ml in the original sample?

Accepted Answer

$$1.2 	imes 10$$^{6}$$ cells/ml$$\ (120\ cells/1 	imes $$10^{-4}$$\ ml = 1.2 	imes $$10^{6}$$\ cells/ml)$$

Question 3

Which of the following best describes the main limitation of the direct microscopic count method?

Accepted Answer

It cannot distinguish between live and dead cells.

Question 4

When using a spectrophotometer to measure bacterial growth, which of the following statements is correct?

Accepted Answer

Absorbance is directly proportional to cell concentration.

Question 5

A student prepares a series of five 1:10 serial dilutions from a bacterial culture. If the original culture contains $$1 	imes 10$$^{8}$$ cells/ml, what is the expected concentration in the final tube?

Accepted Answer

$$1 	imes 10$$^{3}$$ cells/ml

Question 6

Why is it important to mix each dilution tube thoroughly before transferring to the next tube in a serial dilution series?

Accepted Answer

To evenly distribute the bacteria, ensuring accurate and reproducible dilutions.

Question 7

Which plate in a serial dilution series should be used to calculate the number of colony forming units (CFUs) per ml in the original sample?

Accepted Answer

The plate with 30-300 colonies (countable plate).

Question 8

A student spreads 0.1 ml of a $$10^{-4}$$ dilution onto an agar plate and counts 50 colonies after incubation. What is the CFU/ml in the original sample?

Accepted Answer

$$5 	imes 10$$^{6}$$ CFU/ml

Question 9

Which of the following is a disadvantage of using turbidity to estimate bacterial concentration?

Accepted Answer

It cannot distinguish between live and dead cells.

Question 10

What is a colony forming unit (CFU) and why is this concept important in plate counts?

Accepted Answer

A CFU is a group of cells that can form a colony; it is important because some bacteria exist in clusters or chains.

Question 11

If a culture tube appears less turbid than another, what does this indicate about the bacterial concentration?

Accepted Answer

It has a lower bacterial concentration.

Question 12

Which of the following best describes the relationship between absorbance (A) and transmittance (T) in a spectrophotometric measurement?

Accepted Answer

$$A = -\log_{10}(T)$$

Question 13

A plate count yields 75 colonies from plating 0.1 ml of a $$10^{-5}$$ dilution. Calculate the CFU/ml in the original sample.

Accepted Answer

$$7.5 	imes 10$$^{7}$$ CFU/ml

Question 14

Which of the following is NOT a method for enumerating microorganisms?

Accepted Answer

Gram staining

Question 15

Why is the range of 30-300 colonies considered optimal for a countable plate in viable plate counts?

Accepted Answer

Below 30, statistical error is high; above 300, colonies may merge and be difficult to count accurately.

Question 16

A student forgets to mix a dilution tube before transferring to the next tube in a serial dilution series. What is the most likely consequence?

Accepted Answer

The final concentration of bacteria in subsequent tubes will be unpredictable and likely inaccurate.

Question 17

Which of the following statements about the spread (spin) plate method is correct?

Accepted Answer

It requires the sample to be dilute enough to allow for isolated colonies to form.

Question 18

Which of the following is an advantage of the plate count method over direct microscopic count?

Accepted Answer

It counts only viable (living) cells.

Question 19

If you observe a confluent lawn of growth on a plate, what does this indicate about the sample and the plating method?

Accepted Answer

The sample was too concentrated to allow for isolated colonies.

Question 20

Which of the following best explains why some bacteria form clusters or chains, affecting CFU counts?

Accepted Answer

Each cluster or chain forms a single colony, so CFU counts may underestimate the actual number of cells.

Microbiology Lab 4 Study Guide

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