Understanding Bacteriophages in details

 A Bacteriophage possesses viral proteins, disrupting its host cell, i.e., Bacteria. It first attaches to the susceptible bacterial cell and injects its genetic material into the host cell. The viral genome within the host cell takes over the cell machinery and uses the bacterial components to release viral proteins.

Eventually, the viral proteins assemble to form virus particles that release the host cell by causing cell lysis. In some cases, the viral genome remains integrated with the bacterial genome. Bacteriophage was first studied by F.W. Twort (1915). Later the term bacteriophage was given by Felix de Herelle (1917).

After that, he found that the bacteriophage is helpful for the patient suffering from dysentery caused by Bacillus dysenteriae and introduced Phage therapy. In this context, we will talk over the bacteriophage's classification, features, morphology, and life cycle.

Content: Bacteriophage

  1. Definition
  2. Classification
  3. Characteristics
  4. Structure
  5. Life cycle
    • Lytic Cycle
    • Lysogenic Cycle
  6. Significance

Definition of Bacteriophage

If we break the term bacteriophage, we can remember the meaning, viz. Bacteria means bacteria, and Phage means invaders or eaters. Therefore, a bacteriophage can define as a virus, which invades the bacterial cell machinery. Bacteriophages show a parasitic relationship with their host cell, i.e., bacteria. It not only infects the host (bacteria) but also controls the host's machinery. These are ubiquitous in nature.


The bacteriophages are classified by ICTV (International Committee on Taxonomy of Viruses) based on genome and morphology.
Based on the genome: Bacteriophage can be classified into the following four types:

  • ds-DNA
  • ss-RNA
  • ds-DNA
  • ss-RNA
Bacteriophage familyGenomeExamplesDiagram
MyoviridaeGenome: Linear, ds-DNAColiphage T2, T4, T6myoviridae
StyloviridaeGenome: Linear, ds-DNAColiphage T1, T5styloviridae
PedoviridaeGenome: Linear, ds-DNAColiphage T3, T7pedoviridae
CorticoviridaeGenome: Circular, ds-DNA
Pseudomonas phage MP2corticoviridae
TectiviridaeGenome: Linear, ds-DNA
Phage PRD1tectiviridae
PlasmaviridaeGenome: Linear, ds-DNA
Phage MV-L2plasmoviridae
MicroviridaeGenome: Circular, ss-DNA
Coliphage ɸX174microviridae
InoviridaeGenome: Circular, ss-DNA
Coliphage fdinoviridae
CystoviridaeGenome: Segmented, ds-RNA
Phage ɸ6cystoviridae
LeviviridaeGenome: Linear, ss-RNA
Phage MS2leviviridae


  • A bacteriophage is smaller than a bacteria.
  • The genetic material of bacteriophages can be either DNA or RNA and linear or circular.
  • Its genome size is about 49 kb.
  • The bacteriophage size ranges from 25-200 nm in length.
  • Bacteriophages can be either infectious or non-infectious to the host cell.

Structure of Bacteriophage

The morphology of bacteriophage includes the following components:
bacteriophage structure


  • It is elongated and hexagonal in shape.
  • The head of the bacteriophage possesses a prismoid structure.
  • It is surrounded by an envelope called a capsid.


  • It is produced by the identical protein subunits called capsomeres.
  • It contains around 2000 capsomeres.

Genetic material:

  • It is 50 nm long and can be either DNA or RNA.
  • The structure of genetic material can be linear or circular.
  • It is tightly packed inside the head.


  • It is also called a collar, which connects the head and tail.
  • It possesses a circular plate-like structure.


  • It resembles a hollow tube.
  • A tail is surrounded by a protein sheath.


  • It is composed of around 144 protein subunits.
  • The sheath of the bacteriophage is highly contractile.
  • It contains 24 rings.

Base plate:

  • It is hexagonal in shape.
  • The base plate is present at a distal end.

Tail fibres:

  • These are attached to the base plate.
  • It appears long and thread-like filaments.
  • Tail fibres induce host specificity, or they are host-specific.
  • They are generally found 6 in number.
  • Size: 130x2nm


  • It is also called a tail pin.
  • Spikes recognize the receptor sites of the host cell.

Life Cycle of Bacteriophage

Lytic and lysogenic cycles are the typical phases of the bacteriophage life cycle.

two kinds of life cycle

Lytic Cycle

It is also called the Virulent or Infectious cycle. It includes the lytic phages. The phages of a lytic cycle can infect or kill the host cell, due to which the cycle is known as a virulent phase. It includes the following steps:

  1. Attachment
  2. Adsorption
  3. Penetration
  4. Replication
  5. Assembly
  6. Cell lysis

Attachment: In this step, a tail pin or spike recognizes the receptor sites on the host bacterial cell and attaches to it. After that, the tail fibres of the bacteriophage attaches to the surface of the bacteria.
lytic 1

Adsorption: In this step, the amino groups of the host cell react with the carboxyl group of the bacterial cell and vice versa. After the reaction, the tail fibres secrete a lytic enzyme. This enzyme creates a hole in the host cell through which a viral genome moves into the host cell cytoplasm.lytic B

Penetration: This step is also known as "Injection," in which the phage genetic material penetrates the host's genetic material.

Replication or biosynthesis: After the penetration, the mRNA moves out of the nucleus to the cytoplasm, and the viral genome degrades the host genetic material. Then mRNA transcribes and translates to form protein capsid, which leads to the biosynthesis of other components.
lytib C

Assembly: It is also referred to as "Maturation." At this stage, all the bacteriophage components assemble to produce new daughter or progeny virions.

Lysis: At this stage, the virus kills the bacterial cell by cell lysis and releases about 100-200 progeny virions.

Lysogenic Cycle

It is also termed as a Temperate or Non-infectious cycle. The phages that participate in this cycle are known as "Lysogenic phages." These phages do not kill or infect the bacterial cell, due to which a lysogenic phase is considered a non-infectious phase. It includes the following steps:

  1. Attachment
  2. Penetration
  3. Incorporation of genetic material
  4. Replication
  5. Cell division
  6. Induction

Attachment: In this step, lysogenic Phage first recognizes the receptors site of the host through their spikes. After recognition, tail fibres attach to the host cell surface.
lysogenic 1

Penetration: It is also the same as the lytic phase, in which the tail fibres release a lysogenic enzyme to create a pore through which a viral genome goes into the cytoplasm of the host cell.
lysogenic b

Incorporating genetic material is also called "Integration of genetic material." At this stage, phage DNA contains the host genetic material in genetic material, which forms a complex (Prophage). Prophage is a temperate phage that remains non-active, i.e., it cannot produce new progenies.

This step simply recombines Phage and the host's genetic material, which does not involve the transcription and translation processes.


Replication: During favorable conditions, prophage replicates when the bacterial genome replicates and passes onto the daughter cells.

Cell division: In this stage, a cell divides into two identical daughter cells.


Induction: It has two conditions:

  • The prophage can remain in the dormant state inside the host cell.
  • Secondly, prophage can continue the lytic cycle by inducting UV-rays or heat treatment.


  • It helps in the identification, classification, and detection of pathogenic bacteria.
  • It acts as a biocontrol agent by killing the bacteria involved in soil and water pollution.
  • Studying the concept of evolution also acts as a modal organism.
  • Bacteriophages have significant use in genetic engineering.
  • In space microbiology, it is used as a radiation detector.
  • It is significantly used to treat many diseases caused by bacteria, also known as phage therapy.
  • Bacteriophages play a central role in controlling bacterial plankton growth.
  • In horticulture, these are used in a spray to protect plants and vegetables.
  • Also, acts like biocides such as disinfectants to clean up the environmental surfaces. E.g., In hospitals.

Therefore, a bacteriophage shows a wide range of ecological, molecular, and biomedical significance.

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