Phage Biology

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Phage Biology Phage are viruses that grow on
bacterial cells bacteriophage bacteria eater.
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There are 3 basic types of phage structure.
300nm in length
Head diameter 50nm
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Phage can be detected (and counted) by a plaque
assay. 1. Mix phage suspension and bacterial
cells. 2. Add molten cooled soft agar. 3. Pour
on nutrient agar plate.
Phage particle
Top agar
Base agar
Plaque
Lawn
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Phage can replicate in 2 ways (1) by
multiplying in a host cell (lytic growth) (2) by
integrating their genome into the bacterial
chromosome (lysogenic growth).
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Phage adsorbs to specific receptor on host cell
surface and injects DNA.
Lysogenic growth
Lytic growth
Induction
Phage DNA integrates into bacterial chromosome.
Phage multiplies and bursts host cell.
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A phage genome inserted into a bacterial
chromosome is called a prophage. A bacterium
carrying a prophage is called a lysogen. A
repressor switches off phage genes. A lysogen is
immune to superinfection by the the same type of
phage. Prophage induction results in phage
multiplication and host cell lysis.
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Virulent phage are only capable of lytic
growth. Temperate phage can undergo lytic or
lysogenic growth. 90 of all known phage are
temperate.
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Transduction transfer of bacterial DNA from one
cell to another by a phage particle.
leu thr -
leu - thr
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Phage conversion Some prophages carry genes that
convert the host bacterium (confer new
properties). Many bacterial protein toxin genes
are carried by converting phages Bacterial
species Toxin
Corynebacterium diphtheriae Diphtheria
toxin Vibrio cholerae
Cholera toxin Streptococcus pyogenes
Erythrogenic toxin Escherichia coli O157
Vero cytotoxin Clostridium botulinum
Botulin (neurotoxin)
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Host restriction and modification Restriction
enzymes - endonucleases that cut specific
sequences in DNA -restrict the growth of
bacteriophage by degrading phage
DNA. Modification enzymes protect host DNA by
methylating sensitive sites
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Type I restriction enzymes bind to specific
sites, migrate along DNA for 1000 to 5000 bases
then cut randomly. Type II restriction enzymes
cut specific sites e. g. Eco RI
------GAATTC------ ------CTTAAG------
---G AATTC---
---CTTAA G---
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Eco RI methylase modifies these sites.
------GAATTC------ ------CTTA
AG------
Methylated adenine base
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Type III restriction enzymes bind to specific
sites but cut DNA 24 to 26 bases
downstream. A few phage genomes escape
destruction. The phage DNA is then modified and
progeny are resistant to the restriction enzyme.