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Summary Bacterial toxins
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Molecular Basis of Bacterial Infections Evelien FloorBacterial toxins
Introduction to bacterial toxins
There are many different types of toxins. Toxins are not required for growth but are important in
vivo. Knocking out a toxin will therefore not affect bacterial growth. Toxins are virulence factors;
introduction of a toxin can turn harmless bacteria into pathogens. There are many different toxins
which genes are carried on mobile elements: plasmids and bacteriophages.
Toxins are dispensable in vitro but essential under certain situations where survival and spread are at
stake. Bacterial mutants lacking toxins are often less virulent in vivo. Toxins often relate to a specific
disease state. Finally, toxins work at extremely low levels, they include the strongest poisons known.
For instance, 1 kg Tetanus, Botulin, or Shiga toxin is enough to kill the entire world population.
There are two types of actions of toxins:
1. Lysis or overactivation of host cells
o Can be accomplished from the outside
2. Stop or interfere with cell growth
o The protein has to enter the cell
Endotoxin
The outer leaflet of Gram-negative bacteria
is covered with LPS or endotoxin. LPS is
released when Gram-negative bacteria lyse.
LPS activates TLR4 which activates the
innate immune system. Only a very little
amount of LPS is required to activate TLR4.
However, a much higher amount is
required in case of a TLR4 knockout. But
much less bacteria are required to cause
death in case of a TLR4 knockout. This is
because TLR4 is important for immune
activation in case of an infection.
LPS is evolutionary conserved and present
on all Gram-negative bacteria, not only in pathogens. LPS alerts the immune system via TLR4,
primarily by induction of cytokines. Too much LPS is toxic and leads to sepsis, which is a life-
threatening inflammatory response. This is why LPS is called a toxin.
Exotoxins
Exotoxins are secreted proteins or extracellular actin toxins. There are two different types of
exotoxins:
1. Type I: activation of intracellular signalling pathways
o Superantigens
2. Type II: membrane damaging (pore forming proteins)
o Cholesterol-dependent cytolins
o Bicomponent toxins
Superantigens (type I)
Superantigens activate T-cells in an antigen independent way. This way, they cause an intense
immune response due to release of cytokines from and proliferation of T cells. This can stimulate a
high percentage of the entire T cell population. Superantigens are active at low concentrations (0.1
pg/ml). Superantigens bind to MHC class II and TCR independent of a signal peptide.
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, Molecular Basis of Bacterial Infections Evelien Floor
Superantigens of Staphylococcus
aureus
S. aureus produces around 25 superantigens
which are structurally related. One of them is
TSST-1 which causes toxic shock syndrome
(fever, hypotension, shock, death). There are
also enterotoxins which play a role in food
poisoning and cause nausea vomiting and
diarrhoea.
Pore forming toxins (type II)
Pore forming toxins bind to specific proteins
or phospholipids in a membrane. They are
secreted as monomeric proteins but will form
a pore via oligomerization. Binding to their
target receptor leads to an increase of local
concentration causing oligomerization. There is a structural division of the secondary structure of the
membrane spanning domain: an alpha helix or beta barrel. There are differences in the size of a pore,
the size also determines whether ions, molecules or proteins can get through.
Targets of pore forming toxins can be sugars, lipids or protein receptors. Most of the time toxins
target specific cell types. Often this targeting is specific because of the use of a co-receptor. Only
when the co-receptor is also present the pore can be formed.
There are two ways of forming a pore in a membrane by toxins. The first way, pore formation takes
place directly inside the membrane. The second way, pre-pore formation takes place first followed by
the insertion of this pre-pore into the membrane.
Pore forming toxins have different effector functions:
Increase of membrane permeability
o Ions, molecules, proteins
o Loss of barrier function
o Cell lysis
Inducing apoptosis
Intracellular survival
Delivery of other virulence factors
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