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Summary CRISPR/Cas and applications
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Molecular Basis of Bacterial Infections Evelien FloorBacterial defence mechanisms against
bacteriophages
Bacteriophages are viruses that infect and replicate in
bacteria. A bacteriophage is the most common and
diverse entity on earth. Infections causes killing of the
cell releasing progeny virus.
The life cycle of a bacteriophage starts with adsorption
and DNA entry. Once in the cell DNA replication is
initiated. Afterwards transcription and translation take
place and new bacteriophages are assembled. At some
point the host cell is going to be lysed and the new
bacteriophages are released.
Defence systems
Different defence systems of bacteria are:
1. Block adsorption
2. Block uptake of DNA
3. Abortive infection
4. Restriction-Modification systems
5. CRISPR/Cas systems
There is a certain co-evolution
between a parasite and a host: for a
species to survive it has to
constantly evolve, which is just
enough to stay in the competition
for survival.
Block adsorption
Adsorption can be blocked with the
use of a modified phage receptor or
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, Molecular Basis of Bacterial Infections Evelien Floor
another molecule that interferes with the bacteriophage. By evolution the entry receptors on
bacteria change. However, the virus can also change its phage binding protein and yet infect the
target.
Adsorption can also be blocked by
masking the receptor with different
compounds such as polysaccharides. Here
are as well countermeasures that
bacteriophages have evolved, for instance
the presence of polysaccharide
lyase/hydrolase to remove the
polysaccharide and yet bind to its
receptor.
A third way of blocking adsorption is
stochastic expression of attachment
factors. Normally there is one receptor for
a certain phage. But the bacterium can for
instance express multiple other receptors
over time to which the phage cannot
bind. However, bacteriophages also have
evolved a broader specificity to those
different receptors.
Block uptake of DNA
A phage wants to be the only phage
infecting a bacterium at that moment.
Therefore, it uses proteins to block the
uptake of DNA of other bacteriophages. This process is called superinfection exclusion (Sie).
Abortive infection
Bacteria are able to sense infection by a
bacteriophage and kill itself to prevent
infection of the rest of the population.
Toxins inside bacteria are bound to
antitoxins so bacterial death is repressed. In
case of infection with a bacteriophage, the
antitoxins are inactivated which causes
bacterial death by for instance blocking
translation.
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