bacteriophage 1

BACTERIOPHAGE BIOLOGY

LECTURE 1

HOZA, A.S

2009

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

• 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

Bacteriophages

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

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

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

Bacteriophages

• T-even phages

Bacteriophages

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

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

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

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

Bacteriophages


– 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

Bacteriophages


– 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

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

Bacteriophages

Bacteriophages


– 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

Bacteriophages

Bacteriophages


– 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

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

bacteriophage 1

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