00:01
Another toxin and a very important one, produced
by Gram negative bacteria is endotoxin.
00:10
Now, if you don’t remember what a gram negative
bacteria is, go back to the introductory bacteria
lecture and take a look. Endotoxin is sort
of like the calling card of gram negative
bacteria, it announces to the host, 'I'm here
and you better watch out'. Endotoxin is nothing
more than lipopolysaccharide, that outer layer
on the outer membrane of a gram negative bacteria.
00:37
So that outer layer is a lipid bilayer, but
the outer layer is unusual, it's not the standard
kind of lipid, it's made of lipopolysaccharide
and that is what is endotoxin. Now despite
the name endotoxin, this toxin is not internalized
into the host cell, it remains extracellularly,
it just happens to be the name that has stuck
over the years. Let’s look in closer at
that outer membrane, that lipopolysaccharide;
here it is in some detail. Remember the very
bottom, there are lipid chains that form the
outer leaflet of that membrane and that component
is called lipid A, that's the active component
of endotoxin, that is what has the biological
effect, and then there are other portions
including the O portion that we talked about,
which extends above, completing lipopolysaccharide.
Lipopolysaccharide is recognized by pattern
recognition receptors located on the surface
of the eukaryotic cell. We all have such receptors
in order for us to recognize what is foreign,
and when foreign molecules are recognized,
the result is a production of cytokines, which
mobilize the immune response, but may also
have detrimental properties. So on the left
of this slide are three pattern recognition
receptors, one of which recognizes LPS, that
portion of the outer membrane of bacteria
that we've just been talking about. Another
one, FLA, recognizes flagellum, the protein
that makes up the flagella of bacteria that
helps them to move. So here's a close-up look
at the pattern recognition receptor and it
again recognizes endotoxin as being present,
initiates cytokine synthesis and depending
on how much endotoxin is present, low or high,
it has different effects on the cell. So you
can imagine that early in infection, it has
one kind of effect, where the endotoxin is
low, and then if the infection proceeds unchecked,
you will have higher concentration of endotoxin,
you will have a very different effect.
02:53
These receptors for endotoxin and other bacterial
products are called innate receptors, they
also sense viral infections as well. So at
low concentrations of endotoxin, there are
a variety of effects, many of which reflect
the attempts at the immune system of eliminating
the bacterial infection, so let's take a look
at some of these. You can see the four different
targets here and the ultimate activities and
effects of endotoxin. At low concentrations,
endotoxin targets Kupffer cells, macrophages
of various sorts, it causes release of cytokines,
when it binds to its receptor cytokines are
produced, and some of the cytokines cause
fever. So endotoxin is well known to be pyrogenic,
fever inducing, the mechanism is by being
recognized by that innate receptor and producing
a cytokine that induces fever. Cytokines,
endotoxin also activates macrophages, it makes
them more phagocytic, it makes them secrete
hydrolases and have just more enhanced killing,
the idea being of course, “there is endotoxin
here, we sensed it, there must be a gram negative,
we are going to kill it, so we are going to
get activated”. At low concentrations, endotoxin
also activates neutrophils, this has an effect
of dilating blood vessels, and allowing immune
cells to come in and clear any infections
that may be present. Endotoxin also activates
B lymphocytes, B lymphocytes of course produce
antibodies and these may be useful for clearing
the infection. Finally at low concentrations,
endotoxin also activates the complement system,
whose goal is manyfold, essentially trying
to get rid of the bacterial infection, complement
can help take up bacteria into macrophages
by opsonization, it can increase capillary
permeability and it can also poke holes into
bacteria, all of this is part of the inflammatory
response to infection. Now in contrast, at
very high levels of endotoxin, it often can
result in shock, fluid loss for example caused
by too many cytokines and disseminated intravascular
clotting or coagulation.