Q: A student performs a plaque assay and counts 50 plaques on a plate inoculated with 0.1 mL of a $$10^{-6}$$ dilution of a phage stock. What is the phage titer in PFU/mL?

The titer is $$5 \times 10$$^{8}$$ PFU/mL. $$\text{PFU/mL} = \frac{\text{Number of plaques}}{\text{Volume plated (mL)} \times \text{Dilution factor}} = \frac{50}{0.1 \times $$10^{-6}$$} = 5 \times $$10^{8}$$

Question 1

Which of the following best describes a key difference between viruses and bacteria in terms of their size?

Accepted Answer

Viruses are generally much smaller than bacteria, often ranging from 20 to 300 nanometers, while bacteria are typically 0.5 to 5 micrometers in size.

Question 2

Which of the following statements about the structure of viruses is correct?

Accepted Answer

A capsid is a protein shell that surrounds the viral nucleic acid, and it is made up of subunits called capsomers.

Question 3

A virus is considered non-living because:

Accepted Answer

It cannot carry out metabolic processes or reproduce independently outside a host cell.

Question 4

Which of the following best describes a naked virus?

Accepted Answer

A virus that lacks a lipid envelope and consists only of a nucleocapsid.

Question 5

Which of the following is true regarding the nucleic acids found in viruses?

Accepted Answer

Viruses may contain either DNA or RNA, but never both in the same virion.

Question 6

A bacteriophage is best described as:

Accepted Answer

A virus that specifically infects bacteria.

Question 7

During the lytic cycle of a bacteriophage, which of the following events occurs last?

Accepted Answer

Lysis of the host cell and release of new phage particles.

Question 8

Phage typing is a technique used to:

Accepted Answer

Distinguish between bacterial strains based on their susceptibility to specific bacteriophages.

Question 9

On a phage typing plate, what does the presence of a clear zone (plaque) indicate?

Accepted Answer

The bacterial cells in that area have been lysed by the bacteriophage.

Question 10

A student performs a plaque assay and counts 50 plaques on a plate inoculated with 0.1 mL of a $$10^{-6}$$ dilution of a phage stock. What is the phage titer in PFU/mL?

Accepted Answer

The titer is $$5 	imes 10$$^{8}$$ PFU/mL.

$$	ext{PFU/mL} = \frac{	ext{Number of plaques}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}} = \frac{50}{0.1 	imes $$10^{-6}$$} = 5 	imes $$10^{8}$$

Question 11

Which of the following best explains why a bacterium may be insensitive to a particular phage during phage typing?

Accepted Answer

The bacterium lacks the specific receptor required for phage attachment.

Question 12

In a laboratory phage typing experiment, which result would indicate that a bacterial strain is sensitive to a specific phage?

Accepted Answer

Clear plaques appear on the bacterial lawn where the phage was applied.

Question 13

Which of the following steps is NOT part of the standard procedure for performing a plaque count?

Accepted Answer

Counting the number of individual bacterial colonies on the plate.

Question 14

A researcher observes that a certain bacteriophage forms plaques only on E. coli strain A, but not on strain B. What does this observation demonstrate?

Accepted Answer

Phage specificity, as the phage can infect only certain bacterial strains.

Question 15

Which of the following is a living characteristic of viruses?

Accepted Answer

They can reproduce, but only inside a host cell.

Question 16

A student is given a plate with a bacterial lawn and several clear spots. What is the most likely explanation for these clear spots?

Accepted Answer

They are plaques formed by lysis of bacteria due to phage infection.

Question 17

Which of the following best describes the nucleocapsid of a virus?

Accepted Answer

It is the combination of the viral nucleic acid and the capsid.

Question 18

A scientist wants to determine the number of infectious phage particles in a sample. Which method should they use?

Accepted Answer

Perform a plaque assay and calculate the PFU (plaque-forming units) per mL.

Question 19

Which of the following is a correct step in calculating PFU/mL from a plaque assay?

Accepted Answer

Divide the number of plaques by the product of the dilution factor and the volume plated (in mL).

Question 20

A medical laboratory is investigating an outbreak of a bacterial infection. They use phage typing to identify the bacterial strain responsible. Which result would most strongly support that two patients were infected by the same strain?

Accepted Answer

Both patients' bacterial isolates are lysed by the same set of bacteriophages, producing identical plaque patterns.

Microbiology Study Guide: Viruses and Bacteriophages

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