Bacteriophage 1


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Bacteriophage 1

  2. 2. Bacteriophages• All viruses are similar in the basic aspectsof multiplication• General properties can best be studied byselecting a technically suitable model• Rapid advances in virology due to focus onbacteriophage
  3. 3. Bacteriophages• Two types of bacteriophages : 1) Virulent phages multiplication of the phage results in cell lysis lytic cycle 2) Temperate phages multiplication of the phage results in cell lysis --> lytic cycle integration of the phage genome in the bacterial genome --> lysogeny
  4. 4. Bacteriophages• T-even (T2, T4, T6) phages of EscherichiaColi• T-even (T2, T4, T6) phages were thought tobe the simplest possible organism• They turned out to be the most complex ofall viruses• Their complexity allowed many discoveries,and the results could be extended to many otherphages
  5. 5. Bacteriophages• T-even phages are made up of a head and a tail – The head contains the ds linear DNA in association with polyamines, several internal proteins and small Peptides – It has the shape of two halves of an icosahedron connected by a short hexagonal prism – The tail consists of a central helical tube (through which the viral DNA passes during cell infection), surrounded by a helical sheath capable of contrction
  6. 6. Bacteriophages• T-even phages are made up of a head and a tail – The sheath is connected to a thin disc or collar at the head end and to a base plate at the tip end – The plate is hexagonal and of complex structure; it has a pin at every corner and is connected to six long thin tail fibers which are the organs of attachment to the host cell
  7. 7. Bacteriophages• T-even phages
  8. 8. Bacteriophages
  9. 9. Bacteriophages• Structure of other bacteriophages – The coliphages T1 and T5 have a sheathless tail – Phages T3 and T7, as well as the Salmonella phage P22, have a short stubby tail which terminates in a structure resembling a base plate with six short fibers – Some small icosahedral phages, such as φX174, have no tail – Some phages, such as M13, have a helical structure
  10. 10. Bacteriophages• Infection of host cells – The first step in infection is a highly specific interaction of the phages adsorption organelle (f.e. tail or tail fibers) with receptors on the surface of the host cell – This interaction leads to attachment of the phage to the cell (adsorption) – Then the DNA is released from the capsid and enters the cell
  11. 11. Bacteriophages• Adsorption – The initially reversible attachment of the phage to the receptors rapidly becomes irreversible --> the phage cannot be washed away – Adsorption can be abolished by bacterial mutations to bacteriophage resistance • B/2 of E. coli is resistant to T2 • These mutations change the receptors • If B/2 is exposed to a large concentration of T2, rare host-range mutants (T2h) can adsorb to the B/2 and initiate normal viral multiplication
  12. 12. Bacteriophages• Viral sites for adsorption – All virions have a specialized structure for adsorption • T-even phages : the tail fiber • Isolated tail fibers adsorb to the same range of bacteria as the bacteriophage from which the tail fibers were derived • Antiserum to the fibers inhibits phage adsorption • Electron microscopy shows that with the T-even the tips of the fibers attach first and reversibly to the cell surface and are followed by the tail pins which attach irreversibly • The adsorbed virion acquires a charateristic position with the head perpendicular to the cellwall
  13. 13. Bacteriophages• Viral sites for adsorption – The host cell receptors are often complex polysaccharides with phage-binding and antigenic specificity • Phages used in Salmonella typing adsorb to various forms of the O Ag • Salmonella phage P22 needs an intact LPS for adsorption • Isolated receptors can bind to the phage tail blocking adsorption of the phage to bacteria • Receptor for bacteriophage Lambda is a mannose transport protein
  14. 14. Bacteriophages• Viral sites for adsorption – The host cell receptors are often complex polysaccharides with phage-binding and antigenic Specificity • Some male-specific coliphages adsorb only to the sex pili of F+ cells. The RNA- containing phage MS2 adsorbs laterally on the entire F pilus,whereas the DNAcontaining phage M13 adsorbs exclusively on the tip of the F pilus
  15. 15. Bacteriophages• Separation of nucleic acid from coat – In 1952, Hershey and Chase showed the separation of the viral nucleic acid from the capsid – They labeled the proteins with 35S or the DNA with 32P – Used the labeled virus to infect bacteria, and exposed the bacteria to violent agitation in a Waring Blendor, which shears the tails of the adsorbed virions
  16. 16. Bacteriophages
  17. 17. Bacteriophages
  18. 18. Bacteriophages• Separation of nucleic acid from coat – The experiment yielded two results : 1) With 35S-labeled phage 75% of the label came off; but with 32P-labeled phage essentially all the label remained with the cells and since it was Dnase resistant, it was within the cells 2) The blended bacteria produced progeny phage as if they had not been blended – These results strongly suggested that phage DNA carries the genetic information of the phage into the cell
  19. 19. Bacteriophages
  20. 20. Bacteriophages• Separation of nucleic acid from coat – Ejaculation of the nucleic acid from the coat can also be elicited with wall fragments instead of cells – The viral DNA is then released into the medium where it is digestible by Dnase – This result indicated that the injection of DNA into the cell does not require energy from the cell
  21. 21. Bacteriophages• Eclipse – After the nucleic acid is injected, the intact cells can produce plaques, but disrupted cells can not – However infectivity reappears later when progeny virus is formed – The temporary disappearance of infectivity, called eclipse, is due to the inability of the naked viral DNA to infect bacteria under ordinary conditions