Bacterial viruses (Bacteriophages)
* Bacteriophages, also called phages, are viruses that infect bacteria.
* They have the most complex capsids found among viruses.
* Phages have an elongated capsid head that encloses their DNA.
* They have a protein tail piece which helps in attachment of the phage to the host and injecting the phage DNA inside the host cell.
* Bacteriophage (bacterial viruses), 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.
* Significance
– Models for animal cell viruses
– Gene transfer in bacteria
– Medical applications
Composition of 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.
Morphology
* Tadpole shaped
*Hexagonal head (ds DNA)
* Tail: hollow core with contractile sheath
* Base plate: tail fibers
Size
* T4 is among the largest phages; it is approximately 200 nm long and 80-100 nm wide.
* Other phages are smaller. Most phages range in size from 24-200 nm in length.
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-
Replicative Cycles of Phages
* Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle.
1. Lytic cycle i.e. Virulent infection
2. Lysogenic cycle i.e. Temperate infection
