Bacteriophages

 Bacteriophages

Bacteriophages, or phages are viruses that infect bacteria. Bacteriophages were discovered independently by Frederick W. Twort and Félix d’Hérelle.  D’Hérelle coined the term bacteriophage, meaning “bacteria eater,” to describe the agent’s bacteriocidal ability.  Phage means ‘to eat’.  Phages occur in nature in close association with bacteria and can be isolated from feces, sewage and other natural sources of mixed bacterial growth.  Thousands of varieties of phages exist, each of which may infect only one type or a few types of bacteria or archaea.

Morphology

Phages consist of nucleic acid surrounded by a protein capsid.

The head consists of a tightly packed core of nucleic acid (DNA or RNA) surrounded by a protein coat or capsid. The size of the head varies in different phages from 28 nm to 100 nm. The nucleic acid may be either DNA or RNA and may be double-stranded or single-stranded. The capsid is made up of subunits known as Capsomeres.  The capsomeres consists of a number of protein subunits called Protomers. 

The tail is composed of a central hollow core or tube, a contractile sheath surrounding the core and a terminal base plate to which tail fibers or prongs (tail pins) or both are attached.  These tail fibers or prongs help the phage to bind to specific receptor sites on the bac­terial surface.

Type of nucleic acid present in the phage varies, some phages have DNA while some other phages carry RNA as genome. 

Bacteriophage T4                          Prescott

There are three basic structural forms of phage: an icosahedral head with a tail, an icosahedral head without a tail and a filamentous form.

       I.            Icosahedral or cubical symmetry – examples are φX174, MS2

    II.            Icosahedral head with a tail or Binal symmetry– examples are T2, T4, T6

 III.            Filamentous or helical symmetry –Examples are M13, fd

Bacteriophages are grouped into seven morphological types.

Type	Shape	Nucleic Acid	Example
A	Hexagonal head, rigid tail with a contractile sheath and tail fibres	Double stranded DNA	T2, T4, T6
B	Hexagonal head, flexible tail, no contractile sheath and may or may not have tail fibres	Double stranded DNA	T1, T5
C	Hexagonal head, shorter tail, no contractile sheath and may or may not have tail fibres	Double stranded DNA	T3, T7
D	Head made up of large capsomeres, no tail	Single stranded DNA	S13, φX174
E	Head made up of small capsomeres, no tail	Single stranded RNA	f2, MS2
F	Filamentous	Single stranded DNA	fd, f1
G	Pleomorphic, No capsid	Double stranded DNA	MV-L2

 

Life cycle of bacteriophages

During infection a phage get attached to a bacterium and inserts its genetic material into the bacterial cell.   After this, the phage usually follows one of two life cycles, lytic (virulent) or lysogenic (temperate).

Lytic phages take over the machinery of the cell to make phage components. They then destroy, or lyse, the cell, releasing new phage particles.

Lysogenic phages incorporate their nucleic acid into the host cell chromosome and replicate with in it as a unit without destroying the cell. Under certain situations, lysogenic phages get induced to follow a lytic cycle.

Lytic Cycle

During Lytic Cycle, the virulent phage replicate through the following stages - adsorption, penetration, transcription, assembly, maturation and release of progeny phage particles.

Example of a virulent phages are T2, T4, T6 phages of E coli

i. Adsorption

Phage particles attach to virus-specific receptors on the host cell by its tail. Adsorption is a specific process and depends on the presence of complementary chemical groups on the receptor sites on the bacterial surface and on the terminal base plate of the phage.

Initial adsorption of phage to the receptor is reversible, since only tips of tail fibers are attached to the bacterial cell surface.  Then the tail pins attach and the adsorption becomes irreversible. 

Adsorption is a very rapid process and it will be complete within minutes under optimal conditions. Any component on the bacterial surface can serve as receptor for some phage. Bacterial receptor may be part of the LPS, flagella, pili, membrane or wall carbohydrates or proteins.  Host specificity of phages is determined at the level of adsorption.

ii. Penetration

After adsorption, most phages inject their nucleic acid into the bacterial cytoplasm and leave their protein cap­sid outside, similar to injec­tion through a syringe.

During penetration, the tail fibers attach firmly to the cell and firmly attach the phage plate to cell wall.  The contractile tail sheath contracts and this will force the hollow interior tail tube into the bacterial cell wall. The phage DNA then passes through the tail tube. The empty phage head and tail remain outside the bacterium. Penetration may be facilitated by the presence of lysozyme on the phage tail that causes localized digestion of cell wall surfaces.    

Some phages such as T1 and T5 do not have contractile sheath and they inject nucleic acid through adhesion sites between inner and outer membranes of bacterial cell wall.  Filamentous and rod shaped bacteriophages enter bacterial cells and then release DNA in to cell.

iii. Transcription - Synthesis of Phage Nucleic Acid and Proteins

The synthesis of the phage components occurs immediately after penetration of the phage nucleic acid.  It occurs through several stages – formation of immediate early, delayed early and late gene products. 

Immediate early phage genes are transcribed by bacterial RNA polymerases.  Examples for the products are Nucleases to break down host DNA and enzymes to alter bacterial RNA Polymerase. 

Delayed early phage genes products include enzymes to produce Phage constituents, Phage polymerase, ligase and RNA polymerase.

Late gene products include structural components for new phage particles – heads, tails, fibers, etc. and phage lysozyme to lyse bacterial cell for releasing mature phage particles.

iv. Assembly and Maturation

After the synthesis of structural proteins and nucleic acids, the phage components begin to assemble into mature phages.  Phage DNA is condensed into a compact polyhedron and packaged into the head and then the tail struc­tures are added. This assembly of the phage compo­nents into the mature phage particle is known as maturation.

v. Release

Release of the mature progeny phages typically occurs by lysis of the bacterial cell. Phage enzymes act on the bacteri­al cell wall causing it to burst or lyse resulting in the release of mature daughter phages.

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Eclipse Phase, Latent Period and burst size

The interval between the entry of the phage nucleic acid into the bacterial cell and the appearance of the first infectious intracellular phage particle is known as the eclipse phase. It represents the time required for the synthesis of the phage components and their assembly into mature phage particles.

The interval between the infection of a bacterial cell and the first release of infectious phage particles is known as the latent period.

The average yield of progeny phages per infected bacterial cell is known as the burst size (100 to 300 phages).

Lysogenic Cycle

The Lytic or Virulent phages produce lysis of the host cell, while the Lysogenic or Temperate phages enter into a symbiotic relationship with their host cell without destroying the host cell. Lambda phage (λ phage) that infect E coli is an example.

When a temperate phage attacks a bacterium, two things may happen.  In some infected cells, the phage multiplies and a lytic cycle occurs.  In most of the other infected cells, the multiplication of the phage does not happen, it is repressed by a repressor protein.  In this situation, after entry into the host cell, the temperate phage nucleic acid gets integrated into the bacterial chromo­some. The integrated phage nucleic acid is known as the prophage.  Here the prophage becomes and behaves like an integral part of the host chromosome.  As the bacterium reproduces, viral nucleic acid also gets replicated along with bacterial chromosome and is transmitted to the daughter cells.  This phenomenon is called lysogeny and a bacteri­um that carries a prophage within its genome is called a lysogenic bacterium or lysogen.

Under certain natural conditions or under artificial stimuli such as exposure to certain physical and chemical agents such as UV rays, hydrogen peroxide and nitrogen mustard, the prophage may become ‘excised’ from the bacterial chromosome and initiates lytic replication. This is known as spontaneous induction of prophage.

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A lysogenic bacterium is resistant to reinfection by the same or related phages. This is known as superinfection immunity.

Temperate phages are commonly found in clini­cal isolates of gram-positive and gram-negative bacteria and in some cases they contribute to the pathogenicity of the bacteria.

The prophage confers certain new properties on the lysogenic bacterium. This is known as lysogenic conversion or phage conversion.

i. Toxin production in Corynebacterium diphtheriae is determined by the presence of the prophage b in it.

ii. Clostridium botulinum types C and D produce toxin only if these are infected with phage CE b and DE b respectively.

iii. Temperate phages of Salmonella modify the antigenic properties of somatic O antigen in Salmonella.

Importance of Bacteriophages

1. They play an important role in the transmission of genetic information from one bacterium to another by the process of transduction, play a role in the evolution of bacterial types and virulence.

3. Phages may be effective in treating bacterial infections, especially the antibiotic-resistant bacteria, known as Phage therapy.

4. Phages are used as cloning vectors in genetic manipulations and for phage typing to discriminate between bacterial strains.

5. They have a role in the control of bacterial populations in natural waters.