# Bacteriophages and Lysogeny

> Bacteriophages and lysogeny for NEET-PG Microbiology: lytic and lysogenic cycles, phage conversion toxins such as diphtheria toxin, and phage typing.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/bacteriophages-and-lysogeny
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Jawetz, Melnick and Adelberg's Medical Microbiology; Brock Biology of Microorganisms
- How to cite: "Bacteriophages and Lysogeny", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/bacteriophages-and-lysogeny

## 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.

1. **Attachment and injection:** the tail fibres bind a receptor on the bacterium, and the phage injects its DNA through the cell envelope.
2. **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.
3. **Lytic replication:** the host machinery makes new phage genomes and capsid proteins, which assemble into progeny.
4. **Lysis:** holin and endolysin enzymes break the cell wall, releasing new phages.
5. **Lysogenic integration:** the phage DNA inserts into the host chromosome as a prophage and is copied with it into each daughter cell.
6. **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.