Download Bacteriophages are viruses that infect bacteria and are

Bacteriophages are viruses that infect bacteria and are among the
most common and diverse entities in the biosphere.
LEARNING OBJECTIVE [ edit ]
Evaluate the complexity of bacteriophages
KEY POINTS [ edit ]
Phages are obligate intracellular parasites that are able to reproduce only while
infecting bacteria. Bacteriophages are comprised of proteins that encapsulate
a DNA or RNA genome.
Bacteriophages occur in over 140 bacterial or archaeal genera. They arose repeatedly in
different hosts and there are at least 11 separate lines of descent. Nineteen families are currently
recognised that infect bacteria and archaea.
Phages are widely distributed in locations populated by bacterial hosts, such as soil or the
intestines of animals. One of the densest natural sources for phages and other viruses is sea
water.
Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of
carrying out both.
To enter a host cell, bacteriophages attach to specific receptorson the surface of bacteria. This
specificity means a bacteriophage can infect only certain bacteria bearing receptors to which they
can bind, which in turn determines the phage's host range.
TERMS [ edit ]
bacteriophage
A virus that specifically infects bacteria.
lysogeny
the process by which a bacteriophage incorporates its nucleic acids into a host bacterium
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Bacteriophages
Bacteriophages (phages) are potentially
the most numerous "organisms" on Earth.
They are among the most common and
diverse entities in the biosphere. They are
the viruses of bacteria (more generally, of
prokaryotes). Phages are obligate
intracellular parasites, meaning that they
are able to reproduce only while infecting
bacteria. Bacteriophages are comprised of
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proteins that encapsulate a DNA or RNA genome, and may have relatively simple or
elaborate structures . Their genomes may encode as few as four genes, and as many as
hundreds of genes. Phages replicate within bacteria following the injection of their genome
into the cytoplasm.
Artistic rendering of a T4 bacteriophage
The structure of a typical myovirus bacteriophage
Phage-ecological interactions are quantitatively vast. Bacteria (along with archaea) are highly
diverse, with possibly millions ofspecies. Phage-ecological interactions are also qualitatively
diverse. There are huge numbers of environment types, bacterial-host types, and also
individual phage types. Bacteriophages occur in over 140 bacterial or archaeal genera. They
arose repeatedly in different hosts and there are at least 11 separate lines of descent. Over
5100 bacteriophages have been examined in the electron microscope since 1959. Of these, at
least 4950 phages (96%) have tails. Of the tailed phages 61% have long, noncontractile tails
(Siphoviridae). Tailed phages appear to be monophyletic and are the oldest
known virusgroup.
Phages are widely distributed in locations populated by bacterial hosts, such as soil or the
intestines of animals. One of the densest natural sources for phages and other viruses is sea
water, where up to 9×108 virions per milliliter have been found in microbial mats at the
surface. Up to 70% of marine bacteria may be infected by phages.
The dsDNA tailed phages, or Caudovirales, account for 95% of all the phages reported in the
scientific literature, and possibly make up the majority of phages on the planet. However,
other phages occur abundantly in the biosphere, with different virions, genomes and
lifestyles. Phages are classified by the International Committee on Taxonomy of Viruses
(ICTV) according to morphology and nucleic acid.
Nineteen families are currently recognised that infect bacteria and archaea. Of these, only
two families have RNA genomes and only five families are enveloped. Of the viral families
with DNA genomes, only two have single-stranded genomes. Eight of the viral families with
DNA genomes have circular genomes, while nine have linear genomes. Nine families infect
bacteria only, nine infect archaea only, and one (Tectiviridae) infects both bacteria and
archaea.
Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of
carrying out both. With lytic phages such as the T4 phage, bacterial cells are broken open
(lysed) and destroyed after immediate replication of the virion. As soon as the cell is
destroyed, the phage progeny can find new hosts to infect.
In contrast, the lysogenic cycle does not result in immediate lysing of the host cell. Those
phages able to undergo lysogenyare known as temperate phages. Their viral genome will
integrate with host DNA and replicate along with it fairly harmlessly, or may even become
established as a plasmid. The virus remains dormant until host conditions deteriorate,
perhaps due to depletion of nutrients; then, the endogenousphages (known as prophages)
become active. At this point they initiate the reproductive cycle, resulting in lysis of the host
cell. As the lysogenic cycle allows the host cell to continue to survive and reproduce, the virus
is reproduced in all of the cell's offspring. An example of a bacteriophage known to follow the
lysogenic cycle and the lytic cycle is the phage lambda of E. coli.
To enter a host cell, bacteriophages attach to specific receptors on the surface of bacteria,
including lipopolysaccharides, teichoic acids, proteins, or even flagella . This specificity
means a bacteriophage can infect only certain bacteria bearing receptors to which they can
bind, which in turn determines the phage's host range. Host growth conditions also influence
the ability of the phage to attach and invade them..
Electron micrograph of Bacteriophages
In this electron micrograph of bacteriophages attached to a bacterial cell, the viruses are the size
and shape of coliphage T1.
Phages may be released via cell lysis, by extrusion, or, in a few cases, by budding. Lysis, by
tailed phages, is achieved by anenzyme called endolysin, which attacks and breaks down
thecell wall peptidoglycan. An altogether different phage type, the filamentous phages, make
the host cell continually secrete new virus particles. Budding is associated with certain
Mycoplasma phages.
Bacteriophage genomes are especially mosaic: the genome of any one phage species appears
to be composed of numerous individual modules. These modules may be found in other
phage species in different arrangements. Mycobacteriophages - bacteriophages with
mycobacterial hosts - have provided excellent examples of this mosaicism. In these
mycobacteriophages, genetic assortment may be the result of repeated instances of sitespecific recombination and illegitimate recombination (the result of phage genome
acquisition of bacterial host genetic sequences).