Question 1

What is the main purpose of performing a serial dilution before plating a bacterial sample on agar?

Accepted Answer

To reduce the concentration of bacteria so that individual colonies can be observed and counted

Question 2

Why is it important to agitate each dilution tube or bottle thoroughly before transferring to the next dilution?

Accepted Answer

To ensure an even distribution of bacteria throughout the diluent

Question 3

A student counts 120 colonies on a plate where 0.1 ml of a $$10^{-6}$$ dilution was plated. What is the CFU/ml in the original sample?

$$	ext{CFU/ml} = \frac{	ext{number of colonies}}{(	ext{Tube dilution})(	ext{volume plated})}$$

Accepted Answer

$$1.2 	imes 10$$^{10}$$

Question 4

Which of the following best describes a 'colony forming unit' (CFU)?

Accepted Answer

A single bacterium or a group of bacteria that gives rise to a visible colony

Question 5

If a plate has 15 colonies after incubation, how should the result be reported?

Accepted Answer

As Too Few To Count (TFTC)

Question 6

Why is it important to use a new sterile pipette for each transfer during serial dilution?

Accepted Answer

To avoid cross-contamination between dilutions

Question 7

A student plated 1 ml of a $$10^{-7}$$ dilution and counted 250 colonies. Is this plate considered statistically valid for CFU calculation?

Accepted Answer

Yes, because it is between 30 and 300 colonies

Question 8

What is the main reason for swirling the agar in a figure 8 pattern after pouring it into the petri plate?

Accepted Answer

To evenly distribute the bacteria throughout the agar

Question 9

If you plated 0.1 ml of a $$10^{-5}$$ dilution and counted 40 colonies, what is the CFU/ml in the original sample?

$$	ext{CFU/ml} = \frac{	ext{number of colonies}}{(	ext{Tube dilution})(	ext{volume plated})}$$

Accepted Answer

$$4.0 	imes 10$$^{7}$$

Question 10

Which of the following is NOT a reason for performing serial dilutions in microbiology?

Accepted Answer

To increase the concentration of bacteria

Question 11

A plate with a $$10^{-8}$$ dilution has 350 colonies. How should this result be interpreted?

Accepted Answer

As Too Many To Count (TMTC)

Question 12

Which step is essential to ensure that the calculated CFU/ml reflects only viable cells?

Accepted Answer

Incubating the plates for 24-48 hours to allow colonies to form

Question 13

If a student accidentally uses the same pipette for all transfers during serial dilution, what is the most likely consequence?

Accepted Answer

Cross-contamination will occur, leading to inaccurate CFU/ml calculations

Question 14

Which of the following best explains why plates are incubated 'bottoms up'?

Accepted Answer

To prevent condensation from dripping onto the agar surface

Question 15

A food sample is tested for bacterial contamination. After serial dilution and plating, the following results are obtained: $$10^{-5}$$ dilution, 1 ml plated, 280 colonies counted. What is the CFU/ml in the original sample?

$$	ext{CFU/ml} = \frac{	ext{number of colonies}}{(	ext{Tube dilution})(	ext{volume plated})}$$

Accepted Answer

$$2.8 	imes 10$$^{7}$$

Question 16

Which of the following is a critical detail when labeling tubes and plates during a serial dilution experiment?

Accepted Answer

Label all tubes and plates before starting the experiment

Question 17

What is the main reason for using a statistically valid range (30–300 colonies) when counting colonies on agar plates?

Accepted Answer

To minimize counting errors and ensure accuracy

Question 18

In a real-world scenario, why might a microbiologist need to quantitate viable cells in a water sample?

Accepted Answer

To assess the safety of the water for drinking

Question 19

If a student observes a lawn of bacterial growth on a plate, what is the most likely explanation?

Accepted Answer

The sample was not diluted enough

Question 20

Which of the following best describes the relationship between the number of colonies on a plate and the original concentration of bacteria in the sample?

Accepted Answer

The number of colonies is directly proportional to the original concentration, adjusted for dilution and volume plated

Serial Dilution and Agar Plating for Quantitation of Viable Cells

Study Guide - Practice Questions

Study Guide - Flashcards