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gram positive staphylococci
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in depth immunology notes made using synchronous, asynchronous and extra reading material
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Gram positive staphylococciOrigins and classification
Origins of staphylococcus
Staphylococcus means grape-like cluster.
There are two strains of staphylococcus: staph aureus and staph epidermis (lives on
our skin, normally not causing harm).
Nomenclature of staphylococcus.
When the grow on blood agar: golden colonies of s. Aureus and white colonies of
s.Epidermis.
Classification of staphylococci
Staphylococcus genus is the grape-like clusters that we see staphylococcus.
Within the staphylococcus genus there are 11 species groups.
Everyone in this 11 species group will always be gram positive cocci. Cocci means
circular.
Gram positive means that they will have a thick peptide glycan wall and they will
become purple when we stain them with our gram staining procedure.
Kingdom: bacteria. Family: staphylococcocea.
Staphylococcus are facultative anaerobes, this means that they normally breath
oxygen. However, when there is no oxygen, they can change to different respiratory
mechanisms, so they can still function without oxygen.
They grow normally between 18-40 degrees.
But in a laboratory, they are placed in an incubator which is 37 degrees as this has
shown to be their optimal temperature.
This organism is also catalase positive- this means that everyone in this family is
positive for the enzyme catalase. This enzyme transfers hydrogen peroxide into
oxygen. In the laboratory we will take a small colony and place it on the slide and
add about 3% hydrogen peroxide, if it is catalase positive the organism should
bubble, however, all species of staphylococci do this and not all species of
staphylococci causes infections.
One way in which we can majorly differentiate between this family is coagulase, this
is an enzyme that normally only s. aureus does possess.
Coagulase
Coagulase is an enzyme which causes the clotting of plasma by converting fibrogen
into fibrin.
In the laboratory we will have a test tube with some plasma in and if they produce
the coagulase, the positive will go very thick and clumpy. However when negative,
the plasma will still be able to move when inverted and will be runny like liquid and
this means it is coagulase negative.
Coagulase negative staphylococci (CNS): over 30+ species: staphylococcus
epidermidis, staphylococcus saprophyticus (found in UTI infections in females).
, Coagulase-positive staphylococci: staphylococcus aureus, staphylococcus
intermedius (only found in mouths of dogs). We do not find this type of bacteria in
humans.
Pathogenicity mechanisms of S. aureus.
S. aureus is known as a true pathogen. This means that this pathogen is always out
to get you. For example, it can live on your skin knowing that it will not be able to
overcome your body, however, it can then go on to another person and infect them.
To be a true pathogen it needs to do 3 things:
It needs to be able to colonise the host and invade that person’s tissues.
It will then need to be able to evade that hosts defences. Because there is no point
trying to get into the host if you are going to be killed by the immune system.
They also need to be able to damage that host through the production of invasins
and exocellular toxins.
Should also be able to acquire resistance to antibiotics.
S. aureus does this through MRSA
Staphylococcus species and how it can do these mechanisms
STEP 1: we need them to colonise the host.
Staph aurous does this specifically through MSCRAMMS.
MSCRAMMS help the s. aureus bind to different parts of our body.
E.g. the fibronectin binding proteins, this helps bind to fibronectin, this is part of our
extracellular surface in healthy endothelial and epithelial cells.
If you are a healthy cell, it will be able to bind to these, normally it will bind to them
and not continue to cause you infections.
If you are a healthy person, it will bind to these and realise that it will not be able to
take down your immune system. So it can live in your nose, in your armpits etc. them
it will pass onto somebody that it knowns it can infect.
We normally do find these in surgical infections, commonly seen in healthcare
professionals.
S. aureus gets into this patient by a clumping factor. This clumping factor will bind to
the expression of fibrin and fibrinogen to promote adhesion of staph aureus to
damaged tissue.
Collagen and elastin binding proteins- promotes adhesion to severely damaged
tissue.
Once the staph aureus has managed to get into our bodies though any of these
routes, STEP 2 it then needs to evade our host defences.
we do have a microcapsule that surrounds our staph aureus.
Staphococcus has a bound coagulase, this changes the fibrinogen to fibrin, so it
causes a clot within the plasma, by having this on the cell membrane for
staphococcus aureus, it allows different bits of plasma to bind to it in forms of clump.
By forming this clump, it protects the bacteria, then the antigens cannot get to it, it
hides its way in this clump.
We also have protein A, staphylococcus epidermidus do not possess protein A.
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