Lysogenic Cycle of Bacteriophages

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.

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.

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.

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.

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.

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.

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.