Lytic or Virulent CycleThe major events involved in the lytic cycle of T -even phages are:(1) Attachment of phage particle to the host.(2) Adsorption of virus particle.(3) Penetration into the host.(4) Replication of viral nucleic acid.(5) Protein synthesis.(6) Assembly of new virions.(7) Release of mature viruses

(1) Attachment to the host: Random collision brings the phage particles in contact with the bacterial cells. The tail plate of the phage attaches to the surface of a susceptible host bacterium along with tail fibres. Specific components of the protein capsid are known to be involved in the process of attachment of the virus to a specific receptor sites of the host bacterial cell. In E.coli such sites are located in outer lipoprotein layer of peptidoglycan layer.During this process,an attachment site on the virus attaches to a complementaryreceptor site on the bacterial cell. This attachment is a chemicalinteraction in which weak bonds are formed between the attachmentand receptor sites. T-even bacteriophages use fibers at theend of the tail as attachment sites. The complementary receptorsites are on the bacterial cell wall

(2) Adsorption: When the contact is made between tail fibres and bacterium, itbecomes unfolded from the tail. Unfolding and release of tail fibres from whiskers aregoverned by certain co-factors for ego tryptophan is needed (1 mgl ml). The whole processof interaction of phage to recognition of specific receptor site is known as landing (A)After landing there starts the second process of adsorption which is known as pinning(B). Before pinning the phage can move attached with tips of tail fibres to cell surfaceuntil it finds the site for pinning of spikes. Pinning is the irreversible process. All theactivities before pining are reversible.VIRUSES 121After pinning the tail sheath contracts and therefore, appears shorter and thicker(C). Sheath contracts by rearrangement of discs from 24 to 12. The phage uses its energyin tail contraction as ATP. The activity of phage contraction is comparable to that ofmicrosyringe (D). The base plate through the centre enlarges after contraction of sheath.After making the contact with a component of plasma membrane, unplugging of phageDNA begins (E). Thereafter, DNA is injected into the cell without requiring metabolicenergy. (F) Phage head and tail remain outside the cell. Such empty protein coats areknown as 'ghosts'.The adsorption of the virus particles is a specific process for which the presenceof some cations is necessary. Both cells and virus particles are negatively charged atpH 7 and positive ions are therefore required as counter ions. The second stage involvesthe interaction of virus particles with specific receptor most of w~ich appears to beglycoproteins. The number of virus particles or infectious units adsorbed per cell isreferred to as the multiplicity of infection (moi). Animal cells are usually capable toadsorbing very large amounts of virus, the number of receptors of most viruses rangesfrom 100,000 - SOO,OOO/cells.(3) Penetration into the host: Adsorption is followed by penetration of nucleic acidof the phage into the bacterial cell. Actually it is like an injection through a syringe.The end plate and the tail fibres are held firmly against the bacterial cell. It causesthe hollow core of the phage tail to pierce through the bacterial cell wall and possiblythe plasma membrane. At this stage the contractile tail sheath functions like a muscle.It derives the energy from ATP present in the tail of the phage. Through the hollowcore of the contracted tail the phage DNA is injected or penetrated into the body ofthe bacterium. Lysozyme, present on the phage tail, may also facilitate the DNA

penetration by producing a hole on the bacterial wall. After the completion of thepenetration process. The empty head and the tail of the phage remain outside thebacterium as the empty shell. Such empty shells are called ghosts.(4) Replication of viral nucleic acids: Successful penetration of DNA will resultin the production of its many replicas. Enzyme are also involved in the replication ofnucleic acid. This replication in the double stranded DNA viruses occur in the nuclearmaterial of the host cell, so does the transcription. Replication process causes severalchanges in the metabolism of the host cell.(5) Protein synthesis: The phage nucleic acid, once inside the bacterial cell, takesover the protein synthesis machinery of the cell. It suppresses the synthesis of bacterialprotein and directs the metabolism of the cell to synthesize the proteins of the phageparticle. This is accomplished by the synthesis of viral specific m-RNA. The replicationof phage DNA follows the semi conservative mechanism. Most of the phage DNA servesas a template in its own synthesis and the rest is used as a template for the synthesisof viral specific m-RNA. The later directs the host cell to synthesize proteins which areused as a subunits (capsomeres) of the protein coat of the phage particle. These proteinare called late proteins as mentioned above. After the penetration of phage DNA someenzyme are first synthesized. These enzymes are necessary for building complexmolecule peculiar to the phage, and represent early proteins. Subsequently, late proteinsappear, which include the protein subunits of head and tail of the bacteriophage.122 TEXT BOOK OF MICROBIOLOGY

(6) Assembly of new virions: The DNA of phage and proteins of its head andtail are synthesize separately in the bacterial cell. The phage DNA is condensed intoa compact polyhedron and packaged into the head. Finally the tail structures are addedand thus new virion is assembled. The process of assembly of different componentsof the phage into the new mature virions is called 'maturation' process on 'morphogenesisprocess'.(7) Release of mature viruses: It is the final step of infection cycle. The entirecycle of phage development is completed in 30-90 min. In an infected bacterium 7-8 phage particles are formed/minute and a total of about 200 phages are formed ina bacterium. During the process of phage replication the bacterial cell wall is weakend.It is facilitated by the enzyme lysozyme, secreted by the phage DNA in the host cell.This enzyme causes the lysis or bursting of cell wall. Such a disintegration or lysisof cell wall releases the mature daughter phages. These liberated phages may againinfect the bacterial cells.Events of life cycle of