Staphylococcus aureus - VIRULENCE FACTORS

VIRULENCE FACTORS

1) Cell Associated polymers:

a. Polysaccharide peptidoglycan 

b. Teichoic acid 

c. Capsular polysaccharide

·         Peptidoglycan - Half of the cell wall weight is peptidoglycan (many cross-linked bridging layers)- makes the cell wall more rigid and gives structural integrity.

·      Teichoic acid - helps in attachment to host mucosa and protects from complement mediated opsonization

         (Opsonization is  an immune process where particles such as bacteria are targeted for destruction by an immune cell )

·         Capsule or polysaccharide slime layer - Facilitates the adherence of bacteria to catheters and other synthetic materials - inhibits opsonization

2) Cell Surface Proteins

a. Protein A – Present on the surface of Staphylococcus aureus (absent in Coagulase negative Staphyloccoccus– CONS)

·         Protein-A detection is one of the specific tests to detect Staphylococcus aureus.

·    Protein A – covalently linked to Peptidoglycan layer

    ·Chemotactic, anti-phagocytic, anti-complementary effects; Induces platelet damage & hyper sensitivity

·       ·    Binds to Fc terminal of Immunoglobulin IgG- Fab portion free to combine with other antigens- Basis of coagglutination reactions, used in streptococcal grouping and gonococcal typing

                                  

       Protein A coated Staphylococci mixed with any IgG antiserum, if mixed with the corresponding antigen, results in agglutination- coagglutination reactions, used in streptococcal grouping and gonococcal typing. 

          Thus Protein-A coated Staphylococci used as non-specific carrier of antibodies directed against other antigens like Streptococci

b. Clumping Factor – Another surface factor, called Bound coagulase

·         Detected by Slide Coagulase test    - Used for identification of  Staphylococcus aureus

    ·    Saline suspension of S. aureus mixed with a drop of human plasma , on a slide- Presence of clumping- positive slide coagulase test

·     ·         Capsulated strains may give negative test (clumping factor enclosed by capsular polysaccharide)

3. Extracellular enzymes

a) Coagulase

·         (Extra cellular Coagulase) - Triggers clotting of human/rabbit plasma- Calcium and other clotting factors not required for coagulase action-  

Coagulase Test – Standard Criterion to identify S.,aureus 

·         Free coagulase secreted into the medium- acts with coagulase reacting factor (CRF) in the plasma. Binds to prothrombin and convert fibrinogen to fibrin

·         Coagulase does not clot plasma of Guinea pigs which lack CRF

      ·  Most human strain strains produce Coagulase type A 

Bound coagulase	Free coagulase
Clumping factor	Coagulase
Heat stable cell constituent	Enzyme secreted into the medium
Independent of CRf	Require coagulase reacting factor (CRF) in the plasma for its action
One type identified	Eight types identified

b) Lipases: Lipid Hydrolases - Digests lipids – allow them to grow on skin surfaces and subcutaneous tissues

c) Hyaluronidase: breaks down Hyaluronic acid –(connective tissue)- enables the bacteria to spread between cells 

d) Staphylokinase: Dissolves fibrin threads in blood clots – allows to free itself from blood clots

e) Fatty acid modifying enzymes and proteases – help in initiation and spread of infection

f) Nuclease – heat stable nuclease

g) β lactamase: breaks down penicillin – resistant to β lactam antibiotics like penicillins and Cephalosporins

h) Protein Receptors- For many mammalian proteins such as fibronectin, fibrinogen, IgG etc. - facilitate Staphylococccal adhesion to host cells and tissues.

4. Toxins

Cytolytic toxins

Enterotoxin

Exfoliative toxin