2. Bacteriophages
• All viruses are similar in the basic aspects
of multiplication
• General properties can best be studied by
selecting a technically suitable model
• Rapid advances in virology due to focus on
bacteriophage
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. Bacteriophages
• T-even (T2, T4, T6) phages of Escherichia
Coli
• T-even (T2, T4, T6) phages were thought to
be the simplest possible organism
• They turned out to be the most complex of
all viruses
• Their complexity allowed many discoveries,
and the results could be extended to many other
phages
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. 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
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. Bacteriophages
• Infection of host cells
– The first step in infection is a highly specific
interaction of the phage's 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. 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. 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. 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. 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. 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
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
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. 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
