Question 1

Which scientist is credited with the discovery of bacteriophages, and what was the significance of this discovery for microbiology?

Accepted Answer

Felix d’Herelle; he demonstrated that viruses can infect bacteria, leading to the concept of bacteriophages.

Question 2

Which of the following best describes the lytic cycle of a bacteriophage?

Accepted Answer

The phage uses host machinery to replicate, assembles new phages, and lyses the host cell to release progeny.

Question 3

During the lysogenic cycle, what happens to the phage DNA?

Accepted Answer

It integrates into the host genome and is replicated along with host DNA.

Question 4

Which of the following is a primary mechanism by which bacteria resist bacteriophage infection at the attachment stage?

Accepted Answer

Modifying or masking phage receptors on the cell surface.

Question 5

What is the function of restriction endonucleases in bacterial defense against phages?

Accepted Answer

They cleave foreign phage DNA at specific recognition sites.

Question 6

In the context of CRISPR-Cas systems, what is the role of the 'spacer' sequences?

Accepted Answer

They serve as genetic memory of past phage infections, guiding Cas proteins to target matching viral DNA.

Question 7

A batch culture of bacteria is infected with a lytic phage. If the burst size is 500 phages per infected cell and there are $$2 	imes 10^2$ infected cells per mL, what is the expected phage concentration after one lytic cycle?

Accepted Answer

$$1 	imes 10^5$ phages/mL

Question 8

Which of the following best describes the three main stages of the CRISPR-Cas immune response?

Accepted Answer

Spacer acquisition, crRNA processing, and effector stage

Question 9

Why do filamentous phages not cause immediate lysis of their bacterial host?

Accepted Answer

They extrude progeny phages through the cell envelope, allowing the host to survive and grow slowly.

Question 10

Which of the following is a real-world application of bacteriophages in medicine or biotechnology?

Accepted Answer

Phages are used as antibiotics to treat bacterial infections resistant to conventional drugs.

Question 11

A bacterial cell has a mutation that prevents methylation of its own DNA. What is the likely consequence when exposed to its own restriction endonucleases?

Accepted Answer

The cell's own DNA will be cleaved by its restriction enzymes, leading to cell death.

Question 12

Which of the following best explains why the gut bacteriophage community is important for human health?

Accepted Answer

Phages regulate bacterial populations, which can influence immune responses and gut health.

Question 13

If a bacterium acquires a new spacer in its CRISPR locus after surviving a phage attack, what is the most direct benefit?

Accepted Answer

It gains sequence-specific immunity against future infections by the same phage.

Question 14

Which of the following is NOT a strategy bacteria use to prevent phage attachment?

Accepted Answer

Synthesizing restriction endonucleases

Question 15

A phage infects a bacterium and its DNA is immediately degraded by host enzymes. Which bacterial defense system is most likely responsible?

Accepted Answer

Restriction-modification system

Question 16

Which of the following equations correctly calculates the burst size of a phage in a one-step growth experiment?

Accepted Answer

$$	ext{Burst size} = \frac{	ext{Total number of phages released}}{	ext{Number of infected cells}}$$

Question 17

Which of the following best describes the effector stage of the CRISPR-Cas system?

Accepted Answer

crRNA guides Cas proteins to recognize and cleave matching phage DNA.

Question 18

Which of the following is a plausible outcome if a bacteriophage infects a bacterium with a defective CRISPR-Cas system?

Accepted Answer

The bacterium is more likely to be lysed by the phage due to lack of adaptive immunity.

Question 19

Which of the following statements about the eclipse period in a one-step growth curve of phage infection is correct?

Accepted Answer

It is the period after phage entry but before new phage particles are assembled and released.

Bacteriophages and Bacterial Defense Mechanisms

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