Bacteriophage: Introduction, Morphology and Life Cycle

In this article we will discuss about Bacteriophage:- 1. Introduction to Bacteriophage 2. Morphology of Bacteriophage 3. Life Cycle 4. Significance.    

Introduction to Bacteriophage:

Bacteriophage (Greek phagein—to eat; bacteriophage, bacteria-cater) are viruses that infect and parasitise bacteria. They cause lysis of bacteria. They are abbreviated as phages.

Twart (1915) and d’Herelle (1917) observed an invisible minute parasite of bacteria that caused lysis of the culture of dysentery bacilli (Sh. shiga) and named it bacteriophage.

Phages occur widely in association with bacteria in the environment (faeces, sewages).

Bacteriophages are highly host specific and, on the basis of phage, typing of bacteria is done.

Morphology of Bacteriophage:

Coli-phages, called T-even phages (T₂, T₄, T₆… series) which attack E. coli are extensively studied. T-even phage is a prototype of bacterial viruses.

The phages are tadpole-shaped and have a head and tail:

1. Head:

It is hexagonal and contains a tightly packed core of nuclei acid (double- stranded DNA) covered by a protein coat (capsid). The head of phage T₄ has a diameter of 65 nm and is 100 nm long.

2. Tail:

The tail is cylindrical and is composed of a central hollow core or tube (7 nm thick) surrounded by a contractile sheath (protein) and a terminal base plate with tail fibres (usually six in number). Each corner of base plate contains a short pin or spike and a 130 nm long tail fibre. The tail of phage T₄ is 100 nm in length and 25 nm in diameter.

Life Cycle of Bacteriophage:

There are two categories of bacteriophages based on their life cycle:

(a) Virulent,

(b) Tem­perate.

(a) Virulent or lytic phage produces lysis of infected cell releasing large number of progeny viruses.

(b) Temperate or lysogenic phage becomes integrated with bacterial chromosome and remains in a dormant state (prophage). The prophage is replicated synchronously with the bacterial chromosome and segregated to the daughter cell during bacterial division without harming the host cell. Phages are usually specific for a few bacterial strains, because of the presence of phage specific receptors.

A. Lytic Cycle:

The replication cycle of virulent phage is divided into five sequential phases :

1. Adsorption,

2. Penetration,

3. Synthesis of phage components,

4. Maturation and assembly, and

5. Release of progeny viruses:

1. Adsorption:

The phage particles come into contact by random collision and a phage attaches to a specific receptor site on the host cell membrane by means of tail fibres.

Adsorption occurs within minutes of contact.

2. Penetration:

After adsorption of phage to bacteria, the tail sheath of phage contracts and the base plate and tail fibres are held firmly against the bacterial cell. As a result the hollow core is pushed downwards through the already weakened part of cell-wall caused by a phage muramidase present on the base plate.

The viral nucleic acid passes down the hollow tube similar to injection through a syringe. The tube does not penetrate the cell wall and the empty head (capsid) and tail remain outside as shell or ghost.

3. Synthesis of phage components:

After the release of nucleic acid into the bacterial cell, the viral genome directs the biosynthetic machinery of host cell to shut down the normal cellular metabolism and to produce components of new virus particles. This is effected by synthesis of specific enzymes (called early proteins) necessary for synthesis of phage components.

Subsequently, late protein, subunits of phage head and tail appear. Some of the components appear in the nucleus and others in the cytoplasm of host cell.

4. Maturation and assembly:

During maturation there is spontaneous assembly of phage DNA head protein and tail protein of phage. Each component of phage nucleic acid acquires a protein coat and finally the tail structures are added forming a virion (infective virus particle).

5. Release:

The progeny phages are rapidly released by the lysis of the infected bacterium. Phage enzyme (probably muramidase) weakens the cell wall during replication of phage. As a result the infected bacterium assumes a spherical shape. Muramidase concentration rises in the late stage of growth cycle, which acts on the already damaged cell-wall causing lysis of cell with release of progeny phage.

Eclipse phase:

The time interval between the entry of phage nucleic acid into the bacterial cell to the appearance of first infectious intra-cellular phage particle is called “eclipse phase”, because viruses cannot be detected within the host cell during this period. The time interval between infection of host cell and sudden increase in extracellular virus is called “latent period”. The duration of eclipse phase is about 15-30 minutes in phages.

B. Lysogenic Cycle:

In lysogenic cycle, the lytic (vegetative) phage becomes integrated with the host cell chro­mosomes and is converted into prophage without lysis of bacterial cell. The prophage may be converted into a virulent vegetative (lytic) phase spontaneously or by physical and chemical agents (UV rays, H₂0₂, nitrogen mustard).

“Prophage” is a latent bacteriophage retaining its DNA. Host bacteria carrying prophage without being lysed by it is called “lysogenic bacteria“. Bacteriophage that parasitises a host bacterium without lysing it is called “temperate phage“. Prophage may be lost during mul­tiplication of lysogenic bacteria by means of excision. The excised prophage can infect other bacterial cells making the lysogenic.

Significance of Bacteriophage:

1. Phage Typing:

On the basis of the susceptibility of different strains of bacteria to different bacteriophages, the typing of bacteria can be done for identification and epidemiological studies. Epidemics of V. cholera, S. typhi, S. paratyphi and Staph, aureus can be detected by phage typing. Phage typing is most useful in intra-species typing of bacteria e.g. to distinguish V. cholera (classical) from El tor biotype V. cholera.

For testing bacteriolysis phage preparations are standardised by titration. Serial dilutions of phage preparations is applied on a lawn culture of a susceptible bacterial strain. The plate is incubated and lysis of bacteria is observed. The “routine test dose” (RTD) of phage is defined as the highest dilution of phage preparation required to produce just confluent lysis.

2. Phage Assay:

When phages are applied on a lawn culture of susceptible strain, clear zones appear after incubation. These clear zones are known as plaques. A single phage produces only one plaque. Phage assay is an useful method in titrating the number of viable phages.

3. Temperate Phage:

Prophage genes may change the properties of host bacterium e.g. toxins of C. diphtheriae and Clostridium are determined by genes carried in prophage DNA. Bacteriophage infection in S. typhi confers a new antigenic surface structure on the host. Such acquisition of new properties by bacteria following phage infection is called “phage conversion“.

In transduction, phages act as carriers of genes from one bacterium to another.