Transcript Viruses of Bacteria

Non-living entities
 Can infect organisms
of every domain
 Commonly referred to
by organism they
infect
 Viruses that infect
bacteria:
Bacteriophage


Virus architecture
› Virus particle called virion
› Consists of nucleic acid
surrounded by protein coat
 Capsid
› Shapes
 Isometric
 Helical
 Complex
› Two types of virion
 Naked – without envelope
 Enveloped – surrounded by
lipid membrane


Viral genome
› DNA or RNA
 NEVER BOTH
› Linear or circular
› Single-stranded or
double- stranded
Replication cycle
overview
› Only multiply inside
metabolizing cell
› Contains information to
make viral proteins,
assure replication and
move in and out of host
cells
› Viruses live in two
phases
 Extracellular phase
 Metabolically inert
 Intracellular phase
 Metabolically active

Penetration
› Entrance of the virus OR its nucleic acid in the
host cell
 Plant and bacteria viruses inject the nucleic acid
into the host through the cell wall
 Animal viruses enter the cell whole
 Phagocytosis
 Membrane fusion occurs with enveloped viruses
 Viruses enter with an uncoating step

Release
› Host cell bursts and releases viruses to the
outside environment
 Viruses are now extracellular
› As viruses leave the host cell, the envelope is
picked up
 The envelope is made of a portion of the host
cell plasma membrane which becomes the
lipid envelope of the virus

Lysogeny
› Replication of a temperate virus
 This is a non-productive cycle
› Lysogeny begins like the lytic cycle
 Adsorption
 Penetration, then;
 Incorporation into genome
› HIV is a lysogenic RNA virus in humans
› λ phage is lysogenic DNA in bacteria

Incorporation
› Prophage: Viral nucleic acid incorporates onto
the host chromosome
› The viral DNA is replicated only when the host
cell replicates
› Cell eventually “pops” off the host chromosome
and returns to the lytic cycle

Lysogenic conversion
› Prophage can confer
new properties on cell
› Phage DNA not
completely
suppressed
 Genes coding for trait
are expressed
 Infected cells have new
characteristics
 Streptococcus
pyogenes
manufactures toxin
resulting in scarlet fever

DNA may be
transferred by a
bacteriophage to a
bacteria in a process
called transduction.

1. Generalized Transduction: In
this type any bacterial gene
can be transferred
 2. Specialized Transduction: In
this type only a few specific
genes can be transferred

Number of different bacteria that phage
can infect termed host range
› Usually limited to single bacterial species for a single
phage

Factors limit host range
› Two most important
 Phage must be able to attach to host receptors
 Restriction-modification system the host cell must
overcome

Receptors on
bacterial surface
› Vary in chemical
structure and
location
 Usually on bacterial
cell wall
› Sites can be altered
by two mechanisms
 Receptor sites can be
altered by mutation
 Lysogenized bacteria
can alter cell surface
 Results in alteration of
receptor site

Restriction-modification system
› Restriction enzyme that codes for endonuclease
› Modification enzyme attaches methyl group to DNA
recognized by restriction enzyme