Learning Objective

Describe how bacteriophage DNA integrates into the bacterial chromosome, explain the concept of a prophage, and understand how lysogenic conversion increases bacterial virulence.

Bacteriophage DNA (Phage = Bacterial Virus)

Bacteriophages are viruses that infect bacteria. Their genetic material may remain inside the bacterial cell in a stable, integrated form.

Prophage (Temperate Phage Integration)

• Temperate phages can enter a lysogenic cycle, where they integrate their DNA into the bacterial chromosome.
• Once integrated, they are called prophages.
• Prophage DNA is usually repressed by a phage-encoded repressor protein, preventing lysis and keeping the phage dormant.

Prophage Gene Expression

While repressed, the prophage:

• It is stably maintained in the bacterial genome.
• Can still direct the synthesis of specific proteins.
• These are often virulence factors that increase bacterial pathogenicity.

Lysogenic Conversion (Critical Concept)

• When a prophage contributes genes that enhance bacterial virulence, this is called lysogenic conversion.
• These genes are NOT from the bacterium; they are phage-encoded.

Classic Examples of Lysogenic Conversion

Phage-encoded toxins include:

High-yield rule:
➡️ Phage = Toxins
➡️ Plasmid = Resistance + Exotoxins (most)

Activity