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Bacteriophage (page 1)
(This chapter has 2 pages)
© Kenneth Todar, PhD
Viruses that attack bacteria were observed by Twort and d'Herelle in
1915 and 1917. They observed that broth cultures of certain intestinal
bacteria could be dissolved by addition of a bacteria-free filtrate
obtained from sewage. The lysis of the bacterial cells was said to be
brought about by a virus which meant a "filterable poison"
("virus" is Latin for "poison").
Probably every known bacterium is subject to infection by one or more
"bacteriophages" as they are known ("phage" for short, from Gr.
"phagein" meaning "to eat" or "to nibble"). Most research has been done
on the phages that attack E. coli,
especially the T-phages and phage lambda.
Like most viruses, bacteriophages typically carry only the genetic
needed for replication of their nucleic acid and synthesis of their
protein coats. When phages infect their host cell, the order of
business is to replicate their nucleic acid and to produce the
protective protein coat. But they cannot do this alone. They require
precursors, energy generation and ribosomes supplied by their bacterial
Bacterial cells can undergo one of two types of infections by viruses
termed lytic infections
infections. In E. coli,
lytic infections are caused by a group seven phages known as the
T-phages, while lysogenic infections are caused by the phage lambda.
T1 through T7, are referred to as lytic phages because they
always bring about the lysis and death of their host cell, the
bacterium E. coli.
T-phages contain double-stranded DNA as their genetic material. In
addition to their protein coat or capsid (also referred to as the
"head"), T-phages also possess a tail and some related structures. A
diagram and electron micrograph of bacteriophage T4 is shown below. The
tail includes a core, a tail sheath, base plate, tail pins, and tail
fibers, all of which are composed of different proteins. The tail and
related structures of bacteriophages are generally involved in
attachment of the phage and securing the entry of the viral nucleic
acid into the host cell.
Electron Micrograph of bacteriophage T4. Right. Model of phage T4. The
phage possesses a genome of linear ds DNA contained within an
icosahedral head. The tail consists of a hollow core through which the
DNA is injected into the host cell. The tail fibers are involved with
recognition of specific viral "receptors" on the bacterial cell surface.
Before viral infection, the cell is involved in replication of its own
transcription and translation of its own genetic information to carry
growth and cell division. After infection, the viral DNA takes over the
machinery of the host cell and uses it to produce the nucleic acids and
proteins needed for production of new virus particles. Viral DNA
the host cell DNA as a template for both replication (to produce more
viral DNA) and transcription (to produce viral mRNA). Viral mRNAs are
then translated, using host cell ribosomes, tRNAs and amino acids, into
viral proteins such as the coat or tail proteins. The process of DNA
replication, synthesis of proteins, and viral assembly is a carefully
coordinated and timed event. The overall process of lytic infection is
diagrammed in the figure below. Discussion of the specific steps
lytic cycle of a bacterial virus, e.g. bacteriophage T4.
The first step in the replication of the phage in its host cell
is called adsorption. The
phage particle undergoes a chance
collision at a chemically complementary site on the bacterial surface,
then adheres to that site by means of its tail fibers.
Following adsorption, the phage injects its DNA into the bacterial
cell. The tail sheath contracts and the core is driven through the wall
to the membrane. This process is called penetration and it may be both
mechanical and enzymatic. Phage T4 packages a
bit of lysozyme in the base of its tail from a previous infection and
then uses the lysozyme to degrade a portion of the bacterial cell wall
for insertion of the tail core. The DNA is injected into the periplasm
of the bacterium, and generally it is not known how the DNA penetrates
the membrane. The adsorption and penetration processes are illustrated
penetration and injection of bacteriophage T4 DNA
into an E. coli cell. T4
attaches to an outer membrane porin protein, ompC.
Immediately after injection of the viral DNA there is a process
initiated called synthesis of
early proteins. This refers to the
transcription and translation of a section of the phage DNA to make a
set of proteins that are needed to replicate the phage DNA. Among
the early proteins produced are a repair enzyme to repair the hole
in the bacterial cell wall, a DNAase enzyme that degrades the host DNA
into precursors of phage DNA, and a virus specific DNA polymerase that
will copy and replicate phage DNA. During this period the cell's
energy-generating and protein-synthesizing abilities are
maintained, but they have been subverted by the virus. The result is
the synthesis of several copies of
the phage DNA.
The next step is the synthesis of late proteins. Each of the
several replicated copies of the phage DNA can now be used for
transcription and translation of a second set of proteins called the
late proteins. The late
proteins are mainly structural proteins that
make up the capsomeres and the various components of the tail assembly.
Lysozyme is also a late protein that will be packaged in the tail of
the phage and be used to escape from the host cell during the last step
of the replication process.
Having replicated all of their parts, there follows an assembly
process. The proteins that make up the capsomeres assemble themselves
into the heads and "reel in" a copy of the phage DNA. The tail and
accessory structures assemble and incorporate a bit of lysozyme in the
tail plate. The viruses arrange their escape from the host cell during
the assembly process.
While the viruses are assembling, lysozyme is being produced as a late
viral protein. Part of this lysozome is used to escape from the host
cell by lysing the cell wall peptiodglycan from the inside. This
accomplishes the lysis of the
host cell and the release of
viruses, which spread to nearby cells, infect them, and complete
cycle. The life cycle of a T-phage takes about 25-35 minutes to
complete. Because the host cells are ultimately killed by lysis, this
type of viral infection is referred to as lytic infection.