Staphylococcus aureus virulence factors, Enzymes, Diseases cause,
Diagnosis, how to prevent the diseases it cause and treatment
● Staphylococcus is a gram- positive cocci
● They occur singly, in pairs and in tetrads
● There cell wall comprises of peptidoglycan and teichoic acid
● They are facultative anaerobes, salt tolerant, hemolytic and are usually catalase
positive.
● It causes pus-forming diseases such as boils, carbuncles, folliculitis, impetigo
in humans.
● It's spread by respiratory passages, by animate or inanimate objects.
Virulent factors
Capsule: Most strains of Staphylococcus aureus have capsules (slime layer-
polysaccharide). S.aureus capsular antigens are surface -associated, limited in
antigenic specificity and highly conserved among clinical isolates. Capsule inhibits
phagocytosis and promotes adherence of the organism to host cells and in prosthetic
devices.
Peptidoglycan: Give rigidity to the cell wall, activates complement. Virulence
potential of S.aureus cell wall - anchored(CWA) proteins covalently attached to the
peptidoglycan is demonstrated in infection models.
Teichoic acid: are major constituents of Staphylococcus aureus. These are two types
of teichoic acid.(TAs).
● Lipo- Teichoic Acid (LTA) anchored in cytoplasmic membrane
● Cell wall Teichoic Acid (WTA): covalently linked to peptidoglycan in the
bacterial cell wall.
Teichoic Acids contribute to staphylococcus adhesion and colonisation, cell division
and biofilm formation. Over-expression of teichoic acid increases the virulence of
S.aureus. In addition, D-alanine residues on teichoic acids contribute to resistance to
cationic antimicrobial peptides such as defensins or cathelicidins and to glycopeptide
antibiotics such as vancomycin.
Staphylococcus Protein A : is a key virulence factor that enables S.aureus to evade
innate and adaptive immune responses. SPA demonstrates multifaceted roles as a
virulence factor.
● Antiphagocytic Function: protein A has an affinity to Fc receptors of the
immunoglobulin. When protein A binds to the Fc end of immunoglobulin, it
makes S.aureus inaccessible to opsonins thus protecting it from being engulfed
and subsequently killed by the immune cells.
● Biofilm formation: protein A facilitates the adherence of S.aureus to Von
willebrand Factors (VWF) - coated surfaces such as endovascular catheters.
● Protein A can also stimulate inflammation in the lung by binding to a receptors
for tumor necrosis factor 1, that is widely distributed on the airway epithelium.
, This interaction has been shown to play a central role in pathogenesis of
Staphylococcal pneumonia.
Fibronectin - binding protein (FnBPs) FnBPA and FnBPB: promote biofilm
formation by clinically relevant methicillin - resistant S.aureus(MRSA) strains.
Collagen - binding protein : one of the cell surface adhesion proteins expressed by
staphylococcus aureus that plays important roles in bacterium - host adherence and
immune reactions.
Extracellular Toxins
1. Cytolytic toxins
● Haemolysis which lyse the RBCs and they are of 4 types(alpha, beta,
gamma and delta)
● Leukocidin: Damage polymorphonuclear leukocytes and necrosis.
2. Enterotoxin
● Heat stable toxin responsible for Staphylococcal food poisoning
● Once formed enterotoxin is not destroyed even if food is heated
sufficiently to kill all viable Staphylococci.
● Nine antigens types(A-J except For F). Enterotoxin F is now known as
Toxic Shock syndrome toxin
● Some strains may produce more than one type of enterotoxin.
3. Exfoliative (epidermolytic toxin) : Two forms of epidermolytic toxins (ETA
and ETB) of S.aureus split human skin at a site in the upper epidermis. Clinical
effects are most common in infants. It is a serine protease which causes
splitting of desmosomes or intercellular bridges in the stratum granulosum.
Epidermolytic toxins are responsible for Staphylococcal scalded skin syndrome
in which the outer layer of epidermis gets separated from the underlying tissue.
4. Toxic shock syndrome (TSST-1): It's a Staphylococcal superantigen(SAg).
Major roles of TSST-1 are as follows:
● Induce cytokine release from macrophage and T. lymphocytes
● Capable of enhancing the toxic effects of endogenous endotoxin
● Produce leakage of endothelial cells.
● Penetrate mucosal barrier
● Staphylococcal superantigen is responsible for almost all menstrual
toxic shock syndrome cases.
Enzymes
A. Coagulase (free - coagulase)
1. Activates a coagulase reacting factor (CRF). normally present in plasma
causing the plasma to clot by conversion of fibrinogen to fibrin
2. May act to coat the bacterial cells with fibrin making them resistant to
opsonization and phagocytosis.
3. Can be detected by tube coagulase test.
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