Bacteriophages PDF

BACTERIOPHAGES DR. R. EMILIN RENITTA Assoc.Professor/DFPT KITS  INTRODUCTION Viruses are small, acellular particles that can replicate only in a host cell. They are obligatory intracellular parasites. They consist of a nucleic acid...

BACTERIOPHAGES DR. R. EMILIN RENITTA Assoc.Professor/DFPT KITS  INTRODUCTION Viruses are small, acellular particles that can replicate only in a host cell. They are obligatory intracellular parasites. They consist of a nucleic acid genome enclosed in a protective protein shell or capsid.  BACTERIOPHAGE Bacteriophage is the virus that infect bacteria. Bacteriophages were discovered by Frederick Twort(1915)and Felix d'Herelle(1917) – coined the term  Bacteriophages have nucleic acid surrounded by capsid. The capsid is made up of capsomeres. Capsomeres consist of protein subunit known as protomeres.  COLIPHAGES Coliphages were discovered by Herelle. Bacteriophages attacking E.coli are called as Coliphages and are designated T-type. These were numbered T1,T2,T3,.....T17 by Max Delbruck(1938). T2,T4,T6... are known as T-even phages. Rest are called as T-odd phages.  CYANOPHAGES Cyanophages were discovered by Schafferman and Morris(1963). First cyanophage was discovered from three blue green algal genera ie, Lyngbya,Phormedium and Plectonema, hence also known as LPP-1.  STRUCTURE OF AT4 PHAGE It contains a head and a tail region. Head is icosahedron with about 2000 capsomeres and contain a 53 micrometer long double stranded DNA in it. Tail is helical symmetry with a core tube(80 angstrom in diameter) surrounded by a protein sheath (consist of 144 subunits arranged in 24 ruins of six subunit each). Sheath is connected to collar at its upper end and a base plate at its lower end.  Base plate is hexagonal with single pin or peg or spike at each corner. A thin tall fiber(1300A long) is also given out from each corner of base plate. Head is prism like hexagonal having 950X650A° dimension. Tail is 950A° in length joined to head by neck and collar. Bacteriophages may be classified into different morphological class A - These are the most complex have hexagonal head and a rigid tail with a contractile sheath and tail fibres. Example - Coliphages T2,T4,T6 B - These are similar to A but lack contractile sheath on the tail and may or may not have tail fibres. Example - Coliphages T1,T5  C - These are same as B type but their tail is shorter than head. Example - Coliphages T3,T7. D - These have head made up of large capsomeres but no tail. Example - ¢X 174 ,S13. E - These are similar to D type instead they have small capsomeres. Example - f2 , MS2.  F - These are filamentous bacteriophage. Example - Coliphages fd , f1. G - These have lipid envelope and are pleomorphic. Example - MV - L2 STEPS INVOLVED IN THE INFECTION OF BACTERIOPHAGE ADSORPTION PENETRATION OR INJECTION ECLIPSE STAGE MATURATION OR VIRUS ASSEMBLY LYSIS AND RELEASE OF NEW VIRUSES  ADSORPTION Attachment of phage to the bacteria is known as adsorption. This occurs when a phage collides with the host bacteria. Phage adsorbs to specific receptors(F pili, lipoproteins,iron transport protein etc.) on the bacteria by the tip of the tail. Example : T4 and T7 Coliphages bind to lipoprotein receptors.  PENETRATION OR INJECTION The tail fibres of the virus bend to bring the spikes and basal plate in contact with the surface of bacterial wall. The sheath is pressed and the viral genome injected to the bacterial cytoplasm. The empty capsid that remains out is called as the ghost or doughnut. This step is active and requires ATP.  THE ECLIPSE STAGE This stage shows following activity in the host cell : - Immunity against infection by same phages by the production of specific enzyme called as repressors. Suppression of all cellular activity of host. Synthesis of proteins coded by phage DNA using hosts amino acid pool. The protein synthesized is called as early proteins.  These proteins are enzymes and are used to seal the hole in cell wall, to destruct the DNA of host. The fresh DNA molecules of phage synthesize late protein which are the viral coat proteins and viral lysozymes. The coat protein are monomer which are assembled into capsid and other viral components.  MATURATION OR VIRUS ASSEMBLY Head and tail are first assembled separately then they are attached to form viral particles. The time period between injection of viral nucleic acid and the first appearance of phage is known as eclipse period. Time period between injection of viral nucleic acid to rupture of host cell is known as latent period.  LYSIS AND RELEASE OF NEW VIRUSES The cell wall burst out at the end of latent period and the phages are liberated. This phenomenon is known as lysis. The number of virions produced per host cell is specific and is termed as burst size. Generally it is 200 - 300.  What are phages good for? They play important role in ecology and evolution of bacteria. Phages assist identification, classification and detection of pathogenic bacteria. They are used in phage therapy(in order to specifically destroy pathogens). They eliminate pathogens like Campylobacter and Listeria in fresh food to reduce food spoilage. THANK YOU