Bacteriophage

Bacteriophage (bacteria-eater), as the name suggests, are the viruses that infect and replicate within bacteria. They are commonly called a phage. They are found everywhere. They contain DNA or RNA in their genome, which is encapsulated in a protein coat. They also infect archaea.

Bactericidal activity of bacteriophage was first observed in 1896 by Ernest Hanbury Hankin in the water of river Ganges, which could kill cholera bacteria.

William Twort discovered bacteriophage in 1915.

D’Hérelle termed them as ‘bacteriophage’ in 1917, as they showed the ability to kill bacteria.

There are several types of phage virus, which infect only certain bacteria specifically.

They act in the same way as antibiotics by disrupting the cell wall of bacteria and have been used for the same. They have the potential to be used against antibiotic-resistant pathogenic bacteria.

Bacteriophage Structure

  • A bacteriophage is made up of a protein coat known as a capsid, which encapsulates the genome. It consists of a polyhedral head.
  • It may be enveloped or nonenveloped and have different shapes such as rod-shaped, filamentous, isometric, etc.
  • The capsid is made up of many capsomeres. The size and shape vary in different species.
  • The genome consists of ss or ds DNA or RNA, which is linear or circular. The genome codes for proteins ranging from 4 to 100. MS2 bacteriophage genome codes for 4 proteins. The largest genome found in a bacteriophage is 735 kbp.
  • The tails may be long or short, contractile or noncontractile. Tail fibres are present, it helps in anchoring the virus to the bacterial cell wall.

Bacteriophage Classification and Examples

Bacteriophages are classified based on their nucleic acid content and morphological characteristics. There are 19 families of bacteriophages found, of which two families are of RNA bacteriophages. The main families and characteristics of bacteriophages are given below with examples.

Examples

Family of the phage virus

Nucleic acid content

Morphological characteristics

T4

(Escherichia virus T4)

Myoviridae

Linear dsDNA, 169 kbp long

Infects E.coli.

Nonenveloped, Icosahedral head, tail is hollow with complex contractile structure and has tail fibres.

It undergoes only lytic life cycle, no lysogenic cycle.

T2

(Enterobacteria phage T2)

Myoviridae

Linear dsDNA, 170 kbp

Infects E.coli.

The famous Hershey and Chase experiment to prove that DNA is a genetic material was done with T2 phage infecting E.coli.

Nonenveloped, icosahedral head and have a contractile tail.

It undergoes a lytic life cycle.

λ

(Coliphage, Escherichia virus lambda)

Siphoviridae

Linear dsDNA, 48502 bp

Infects E.coli.

The virus particle is made up of head, tail and tail fibres.

Nonenveloped and have a noncontractile long tail.

It undergoes both lytic and lysogenic cycles.

Commonly used as a vector in recombinant DNA technology.

M13

(Escherichia virus M13)

Inoviridae

Circular ssDNA, 6407 nucleotides long

Filamentous, nonenveloped.

Infects E.coli. M13 plasmids are used in genetic engineering.

ΦX174 or phi x 174

(Escherichia virus ΦX174)

Microviridae

Circular ssDNA, 5386 nucleotides

Isometric and nonenveloped.

Infects E.coli.

It was the first DNA genome to be sequenced and also the first genome to be assembled in vitro.

It encodes for 11 proteins.

The transcriptome of ΦX174 was generated in 2020.

Qβ, MS2, etc.

Leviviridae

ssRNA, 3500-4200 nucleotides

They are nonenveloped with icosahedral or spherical geometry.

It contains the ‘+’ sense strand of RNA, which codes for four proteins, namely for coat, the replicase, lysis and maturation.

Viral RNA works as mRNA and codes for phage proteins inside the host cell.

They infect enterobacteria, Pseudomonas, acinetobacter, etc.

Φ6, Φ7, Φ8, etc.

Cystoviridae

dsRNA, 14 kbp

They are enveloped with icosahedral or spherical geometry.

They contain protein and lipid outer layers.

They mostly infect Pseudomonas bacteria.

Life cycles of Bacteriophage

There are two ways by which bacteriophages infect the host bacterium.

Lytic Cycle (Virulent infection)

They induce complete lysis of the bacterial cell, which is known as a lytic life cycle. Examples include T2, T4, T6 (T-even phages), they are also known as virulent phages. The bacterial cell is completely destroyed immediately after replication of the viral genome. This type of infection is called virulent infection and it is mostly used for phage therapy.

The lytic cycle has the following steps:

  1. Adsorption- Anchoring of bacteriophage to the bacterial cell wall with the help of tails fibres.
  2. Penetration- The phage DNA gets injected into bacteria.
  3. Replication and synthesis- The bacterial DNA is disrupted and the viral genome takes charge of bacterial machinery. It starts making proteins required for replication and other structural proteins.
  4. Assembly- Phage components are assembled into new viral particles.
  5. Lysis and release- Bacterial cells are lysed and new viral particles are liberated to infect other cells.

Lysogenic Cycle (Temperate infection)

Bacteriophages that undergo lysogeny are known as temperate phages. The viral DNA gets integrated into the host genome and replicates along with the bacterial genome. The integrated viral genome is known as a prophage.

It is relatively harmless and continues to remain in the position until the lytic cycle is triggered. It may be spontaneous or due to certain external conditions such as radiation exposure. Then the prophage becomes active and a lytic cycle initiates resulting in the lysis of the cell wall.

After penetration, the phage DNA gets integrated into bacterial DNA and gets replicated along with the bacterial genome.

As the bacterial genome is inserted into the bacterial genome and bacteria continue to perform the normal activities, the viral genome gets transferred to the progenies as well.

Bacterial cells containing a prophage are called lysogenic cells. The lysogenic cells (having a prophage) may exhibit new properties, e.g. Corynebacterium diphtheriae and Clostridium botulinum, when containing certain prophage DNAs, synthesize toxins, which are harmful.

Examples of lysogenic phage include lambda (λ) phage. Due to the ability to insert their genome specifically and replicate, they are used in genetic recombination.

Importance of Bacteriophage

Bacteriophages are used for various purposes. They are widely used in medical and research.

  • Phage therapy- They are used as antibiotics against bacteria due to the same mode of action.
  • They are used in the food industry to kill bacteria in meat or cheese products.
  • Bacteriophages are used for diagnostic purposes.
  • They act as a model in research and studies.
  • They are used as a cloning vector in genetic recombination technique.

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