Lecture notes
chlamydia
- Module
- Mechanisms of pathology
- Institution
- Aston University, Birmingham (Aston)
in depth immunology notes
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ChlamydiaChlamydia characteristics.
Once chlamydia are obligate intracellular parasites, so they are obliged to live within
cells like viruses, they actually have some features that are similar to viruses whilst
others are not.
These bacteria lack peptidoglycan.
They cannot be filtered by a 0.45 micron filter which is a typical way of removing
bacteria from many solutions. Chlamydia can be very small.
They contain 2 types of nucleic acid, unlike viruses, DNA and RNA.
They possess their own ribosomes so they can make their own proteins and do some
metabolism themselves.
They possess characteristic inner and outer membranes within their cell envelopes.
They have a characteristic inner membrane, plasma membrane and outer
membrane.
The outer membrane is typically gram negative containing lipooligosaccharide.
There is small amounts of peptidoglycan within these organisms, even though they
carry the genetic machinery to be able to make it.
They have a unique, distinctive bacterial life cycle.
Bi-phasic life cycle.
The bacterial life cycle is based on 2 phases, 2 organism types: an extracellular form
and an intracellular form.
The extracellular form are called elementary bodies they are small, they don’t have
any metabolism because they are outside of a cell and they are designed to be
resistant.
The intracellular form are much bigger, they are called the reticulate body, and these
replicate, so they are much more metabolically active and because of that, much
more sensitive to therapy with chemotherapeutic agents
Small electron dense elementary body, it is very dense because the DNA is
condensed, and this means that it is very resistant to any kind of degradation.
There are also reticulate bodies which are much more relaxed because they have
relaxed nucleic acid within the cell.
Infection life cycle key events
The life cycle takes somewhere between 48-72 hours.
It starts with an elementary body, which is an infectious extracellular form, binding
and being taken up into a phagosome.
To then convert and differentiate into a reticulate body within the endosome.
To then divide and divide further and then begin within that inclusion body to
become re-differentiated to an elementary body and then to be released.
This release can happen either by extrusion of the inclusion body or by lysis of the
host cell.
Notably, when the reticulate body is dividing, in some cases it can form a persistent
form of chlamydia which can be problematic to treat and can be reactivated later in
life.
Growth cycle.
, The growth cycle is very characteristic.
You will be able to see a brown iodine-stained inclusion body, the nucleus is shown
in blue.
This is very characteristic of a chlamydial infection.
The phagosomes fuse with each other to make a single intracellular inclusion body
and uses lots of membrane from the various membrane stores within the host cell.
Chlamydia characteristics.
Chlamydia have a very small genome.
They are parasites so they rely on the host cell for lots of their energy, and nutrients.
This means that they don’t need the genetic material to produce their own synthetic
machinery to make the various different factors that it might need to survive.
Chlamydia-endotoxins.
The cell envelope:
The plasma membrane, then the peptidoglycan, this layer is relatively absent within
chlamydia.
And then we have the outer membrane, and the outer membrane has within it a
lipooligosaccharide, which is a lipid A membrane anchor, with a limited number of
core sugars.
Then lipooligosaccharide of chlamydia is all that the chlamydia have, they do not
have the repeating O antigens outside, which would typically make the gram-
negative organism a lipopolysaccharide.
Chlamydia have the smallest naturally occurring LPS.
c.trachomatis structure.
The structure contains a number of proteins within the envelope.
We have outer membrane proteins (OMPS)
Outer membrane protein 1 is known as major outer membrane protein.
These form trimers which provide a porin structure through the cell membrane to
allow important nutrients to be able to pass in.
These proteins have variable segments which are highly susceptible to mutation.
These variable segments may be a way of avoiding the immune system.
If you have antibodies to the major outer membrane proteins, particularly at the
variable sequences, they can protect against infection.
Suggesting that they are also involved in infection, and also involved ideally in
evading the immune response, if you are trying to survive as a bacterium.
Outer-membrane protein-2 (OMP2)
This is a cysteine rich protein, the significance of cysteine is to form disulphide bonds
and that can help to give structure and rigidity to the particle, this is important in the
organism that does not have the typical scaffolding that bacteria do because it lacks
the peptidoglycan.
We also have other outer membrane proteins, such as the polymorphic outer
membrane proteins.
Polymorphic means multiply shaped and these, because they change a lot are able to
help in the evasion of the immune response.
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