Question 1

How does the lysogenic cycle differ from the lytic cycle of bacteriophages?

Accepted Answer

In the lysogenic cycle, phages do not immediately kill their host cell. Instead, phage DNA integrates with the bacterial genome, forming a prophage that replicates passively with host DNA. Unlike the lytic cycle, which produces new phages and destroys the cell, lysogenic infection allows bacteria to survive and reproduce while carrying dormant phage DNA.

Question 2

What is a prophage and how does it form?

Accepted Answer

A prophage is the integrated copy of phage DNA within a bacterial genome. It forms when phage DNA recombines with the host chromosome after injection into the cell. The prophage remains inactive and is replicated along with bacterial DNA during cell division, allowing the phage genetic material to persist across generations without producing new viral particles.

Question 3

What is lysogenic conversion and why is it medically significant?

Accepted Answer

Lysogenic conversion occurs when prophage genes alter the infected bacterium's phenotype, often encoding virulence factors that increase pathogenicity. Normally non-pathogenic bacteria become highly virulent through this process. Examples include Clostridium botulinum, Corynebacterium diphtheriae, and Vibrio cholerae, which cause disease only when infected by specific phages carrying toxin genes.

Question 4

How does Shiga toxin production relate to E. coli O157:H7 infections?

Accepted Answer

E. coli O157:H7 carries a prophage encoding Shiga-like toxin (Stx), which causes intestinal bleeding and kidney failure. During the lysogenic cycle, Stx is not produced and bacteria remain harmless. However, certain antibiotics can trigger prophage induction, entering the lytic cycle and causing Stx production, complicating treatment of these dangerous infections.

Question 5

Can prophages reactivate and re-enter the lytic cycle?

Accepted Answer

Yes, prophages can reactivate and enter the lytic cycle in response to perturbations like DNA damage or spontaneously without external triggers. Once reactivated, the prophage begins producing new phage particles and eventually lyses the host cell. This reactivation is particularly problematic in medical settings, where it can be triggered by antibiotic treatment.

Question 6

Why do large numbers of bacteriophages favor the lysogenic cycle?

Accepted Answer

When bacteriophage populations are large, competition for host cells intensifies. The lysogenic cycle allows phages to persist without killing hosts, ensuring long-term survival and replication of phage DNA. This strategy preserves the host population and provides a selective advantage over the lytic cycle in high-density phage environments.

Question 7

What challenges does lysogenic conversion present for treating bacterial infections?

Accepted Answer

Lysogenic conversion complicates treatment because prophage genes encode virulence factors that make bacteria pathogenic. Standard antibiotics may inadvertently trigger prophage induction, causing the lytic cycle and toxin production. Current research explores novel therapies that prevent prophage initiation to avoid exacerbating infections during antibiotic treatment.

Cycle lysogénique des bactériophages

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