Bacteriophages and Lysogeny

Prepared and reviewed by the PrepElephant Academic Review Team

First published 14 Oct 2026 · Spot an error?

Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Jawetz, Melnick and Adelberg's Medical Microbiology; Brock Biology of Microorganisms

On this page

Direct answer
What you must remember
From attachment to lysis or lysogeny
Where the exam tries to catch you
Frequently asked questions
Related topics

Direct answer

A bacteriophage is a virus that infects bacteria and can follow a lytic cycle that kills the host or a lysogenic cycle that integrates into the host genome as a prophage. In lysogeny the prophage is passed to daughter cells and can convert the host by adding genes such as toxins. Diphtheria toxin, for example, is encoded by a phage gene, so only lysogenic strains of Corynebacterium diphtheriae produce it.

What you must remember

Lytic cycle: attachment, injection, replication, assembly and lysis, with holin and endolysin releasing the progeny.
Lysogenic cycle: the phage genome integrates into the host chromosome, and a repressor keeps the lytic genes silent.
Lambda phage: the classic temperate phage, integrating at a specific site near the gal and bio genes of E. coli.
Induction: DNA damage, such as UV light or mitomycin C, triggers the SOS response and excision of the prophage.
Lysogenic conversion: prophage genes add a new trait, such as toxin production, to the host.
Diphtheria toxin: encoded by the tox gene of corynephage beta, so only lysogenic strains of C. diphtheriae produce it.
Erythrogenic toxin: the SpeA toxin of Streptococcus pyogenes is phage-encoded and causes the rash of scarlet fever.
Phage typing: strains are grouped by their pattern of lysis with a standard set of phages, used in outbreak tracing.

From attachment to lysis or lysogeny

A phage makes its decision after entering the cell, and the same genome can follow either path. The steps below trace that decision in a temperate phage such as lambda.

Attachment and injection: the tail fibres bind a receptor on the bacterium, and the phage injects its DNA through the cell envelope.
Decision point: phage regulatory genes choose between the two paths, and the outcome depends on how many phages infect the cell and on the host's condition.
Lytic replication: the host machinery makes new phage genomes and capsid proteins, which assemble into progeny.
Lysis: holin and endolysin enzymes break the cell wall, releasing new phages.
Lysogenic integration: the phage DNA inserts into the host chromosome as a prophage and is copied with it into each daughter cell.
Induction: DNA damage activates the SOS response, and the prophage excises and returns to the lytic cycle.

Lysogeny is more than a quiet state, because the prophage can change the phenotype of the host. This is why phage-carried toxin genes can turn a harmless strain into a dangerous one. A close relative of a pathogen may differ from it by a single prophage.

Where the exam tries to catch you

Phage questions test the cycle, the conversion event and the typing method.

Trap: a lysogenic phage always kills its host Correct: in lysogeny the prophage stays silent and replicates with the chromosome, and lysis happens only after induction.
Trap: diphtheria toxin is encoded by the bacterial chromosome Correct: the tox gene is carried by corynephage beta, so only lysogenic strains of C. diphtheriae produce the toxin.
Trap: phage typing is equally useful for all bacteria Correct: it is most useful in epidemiology for organisms such as Staphylococcus aureus and Salmonella Typhi, where phage patterns track outbreak strains.

Frequently asked questions

What is the difference between the lytic and lysogenic cycles of a phage?

A lytic phage replicates inside the host and lyses the cell to release progeny within one cycle. A lysogenic phage integrates its genome into the host chromosome as a prophage and does not lyse the cell until induction.

What is lysogenic conversion in bacteria?

Lysogenic conversion is the change in host properties caused by genes carried on a prophage. Examples include diphtheria toxin in Corynebacterium diphtheriae and Shiga toxin in E. coli O157.

Which phage-encoded toxin causes diphtheria?

Diphtheria toxin is encoded by the tox gene of corynephage beta, so only lysogenic strains of Corynebacterium diphtheriae produce it. The toxin is an A-B protein that inhibits protein synthesis in host cells.

What is phage typing used for?

Phage typing identifies bacterial strains by their pattern of lysis with a standard set of phages. It is used for epidemiological tracking, for example of Staphylococcus aureus and Salmonella Typhi outbreaks.