Bacteriophages Structure


Bacteriophages(  Bacterial Viruses):

Bacteriophage is a virus that attacks bacteria  and kill them. They are called eater of bacteria/ phages which  is wrong. They actually infect and kill bacteria and hence called virus parasitizing bacteria.  Bacteriophages were discovered  by a British microbiologist Twist ( 1915) and French microbiologist d' Herelle ( 1917) independently, but term bacteriophage was coined by Herelle. It is called coliphage when infects the colon bacterium E. coli; mycophage ( attacking  fungi), zymophage (on yeast), cyanophage ( infecting cyanobacteria). Schlesinger in 1933 proved that phages have DNA and protein. T2 coli phage is widely studied phage. Its name is phago_ virus secundus. It is a naked virus with binal  ( complex) symmetry and tadpole/ lollipop like in structure  having a head and a tail. In between  head and tail,  there is a small neck and a circular  plate ( collar). Head is polyhedral ( hexagonal) with cubical symmetry.  It is 20 sided (Icosahedral) of 95 nm× 65 size. Tail is of 115 nm length  and 17.5 nm width and has helical symmetry. The capsid of head consists  of 2000 capsomeres  and tail 144 capsomeres. The DNA is about 50%  of total virus with 10000 bp and that is why, phages are useful source of DNA for studies.  The ratio of DNA to protein is 50: 50. DNA  is ds, linear,  52000 nm in length and has HMC ( hydroxymethyl cytosine) instead  of cytosine  and lies in the centre of head. This DNA has 60000 base pairs in T2 phage and 166000 in T4 phage. Tail has a core tube enclosed  by contractile  tail sheath of protein. The capsomeres in tail are arranged  helically  with 6 capsomeres per turn. The core tube at its  tip contains lysozyme enzyme which makes hole in mucopeptid  cell wall of bacterial  host during infection . A hexagonal base plate bearing 6 tail fibres is found  at the base of tail. These tail fibres, each of 130__ 150 nm in length  and 2nm in diameter,  help in attachment . Tail fibres are absent lambda phage.  Tail, base plate and tail fibres make the phage to look like a landing  module on the moon.

   The bacteriophages  reproduce by phagic ( either lytic or lysogenic) or pinocytic reproduction. 

Bacteriophages Structure:

A well studied bacterial virus, which attacks Escherichia coli,  is commonly  called ' coli phage'. It occurs in several strains. The structure  of a typical coliphage  ( T2 phage particle) show the  following  structure:

It is a tadpole like virus consisting  of a head and a tail  of  a  approximately  equal length. The head is  polyhedral ( hexagonal  prism__ shaped with pyramidal  ends). It has an outer coat of protein enclosing a single  molecule of  DNA.  The DNA molecule is double helix, coiled and long ( approx. 50 ų in length) consisting of polynucleotide. The tail is  much narrow in comparison to head. It is hexagonal  in cross section  kinds of proteinaceous  components  forming a spiral sheath.  The tail sheath  protein is contractile  in longitudinal direction. The tail sheath  bears a disc_  like  hexagonal  plate and its distal end. It bears 6 spikes to which are attached  6 tail fibres. There are about 25  striations across the tail.

What causes Bacteriophages viruses?

Bacteriophages, also known as phages, are viruses that specifically infect bacteria. They are found naturally in various environments, including soil, water, and the human body. Bacteriophages are the most abundant biological entities on Earth and are estimated to outnumber bacteria by at least 10 to 1.

The main cause of bacteriophage viruses is the presence of susceptible bacteria. Bacteriophages co-evolved alongside bacteria and have developed mechanisms to specifically recognize and infect certain types of bacteria. They have evolved to effectively target receptors on the surface of bacteria, allowing them to attach and subsequently inject their genetic material into the bacterial cell.

Just like other viruses, bacteriophages require host cells to reproduce and complete their life cycle. Once inside the bacterial cell, the phage replicates its genetic material and causes the host cell to produce new phages. This eventually leads to the death and lysis (rupture) of the bacterium, releasing numerous progeny phages that can go on to infect other susceptible bacteria.

Overall, the primary cause of bacteriophage viruses is the coexistence of susceptible bacteria and the presence of phages that have specific mechanisms to infect them. The ongoing evolutionary arms race between bacteria and bacteriophages has led to the development of diverse phage types and bacterial defenses, making the study of phages an area of great interest in fields such as microbiology and bioengineering.

What are examples of bacteriophages viruses?

There are many examples of bacteriophages, as they infect a wide range of bacteria. Some notable examples include:

1. T4 phage: T4 is a bacteriophage that infects Escherichia coli (E. coli) bacteria. It is one of the most well-studied bacteriophages and has played a crucial role in the development of molecular biology techniques.

2. Lambda phage: Lambda is a temperate bacteriophage that can infect E. coli. It has a unique lifecycle and can either enter a lytic (causing cell lysis) or lysogenic (integrating its genetic material into the host genome) state.

3. P1 phage: P1 is a bacteriophage that infects various strains of E. coli and other related bacteria. It is often used in molecular biology research, such as in transduction experiments.

4. T7 phage: T7 is a bacteriophage that is specific to E. coli. It is commonly used in molecular biology research, including in the production of recombinant proteins.

5. M13 phage: M13 is a filamentous bacteriophage that infects E. coli. It is often used in phage display experiments to study protein-protein interactions and antibody engineering.

These are just a few examples, and there are numerous other types of bacteriophages that infect different bacterial species. The study of bacteriophages is an extensive field with a wide range of phage diversity.


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