Staphylococcus aureus Virulence Factors

By Acharya Tankeshwar •  Updated: 05/28/22 •  4 min read

Staphylococcus aureus is a notable human pathogen for a variety of infections; suppurative (pus-forming) infections, systemic illness and toxinoses. S. aureus has an extraordinary repertoire of virulence factors that allows to survive extreme conditions in human and promote tissue colonization, tissue damage, and ensues life-threatening systemic infections.


Most strains of S. 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, promotes adherence of the organism to host cells and in prosthetic devices.


Give rigidity to the cell wall, activates complement. Find more about peptidoglycan

MSCRAMM (acronym for “microbial surface components recognizing adhesive matrix molecules”)

Teichoic acid

Teichoic acids are major constituents of Staphylococcus aureus. There are two types of teichoic acid (TAs):

Teichoic acids contribute to staphylococcal adhesion and colonization, cell division, and biofilm formation. Overexpression of teichoic acid increases the virulence of S. aureus. In addition, D-alanine (D-Ala) residues on teichoic acids contribute to resistance to cationic antimicrobial peptides such as defensins or cathelicidins, and to glycopeptide antibiotics such as vancomycin or teicoplaninFind more about Teichoic acids (TA).

Protein A

Protein A demonstrates multifaceted roles as a virulence factor.

Protein A as a multi-functional virulence factor

Fibronectin-binding proteins (FnBPs) FnBPA and FnBPB

Collagen-binding protein

Extracellular Toxins

  1. Cytolytic toxin
    1. Haemolysins :
      1. Lyse RBCs
      2. 4 types i.e., alpha, beta, gamma & delta
    2. Leukocidin:
      1. Damage polymorphonuclear leucocytes and necrosis
      2. Panton-Valentine leukocidin (PVL) is a cytotoxin—one of the β-pore-forming toxins.
  2. Enterotoxins
    1. Heat stable toxin responsible for Staphylococcal food poisoning
    2. Once formed enterotoxin is not destroyed even if food is heated sufficiently to kill all viable Staphylococci
    3. Nine antigenic types (A- J except for F). Enterotoxin F is now known as Toxic shock syndrome toxin (TSST-1)
    4. Some strains may produce more than one type of enterotoxins.
  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 toxin (TSST-1): It’s a staphylococcal superantigen (SAg). Major roles of TSST-1 are as follows:

  1. Induce cytokine release from macrophage and T lymphocytes
  2. Capable of enhancing the toxic effects of endogenous endotoxin
  3. Produce leakage of endothelial cells
  4. Penetrate mucosal barrier
  5. Staphylococcal superantigen is responsible for almost all menstrual toxic shock syndrome  cases
Mechanisms by which S.aureus subverts innate immune defenses. Source


  1. 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 opsonisation and phagocytosis
    3. Can be detected by tube coagulase test
  2. Staphylokinase (fibrinolysin):
    1. Fibrinolytic activity
    2. Antigenically and enzymatically distinct from that produced by Streptococcus
    3. Breaks fibrin clot and allow the spread of infection to contiguous tissues
  3. Hyaluronidase: hydrolyses hyaluronic acid present in the intercellular ground substance of connective tissues thus facilitating the spread of the organism to adjacent areas
  4. Deoxyribonuclease – degrades DNA
  5. Lipase – degrades lipids
  6. Phospholipases – degrades phospholipase
  7. Proteases – causes proteolysis

References and further reading

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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7 responses to “Novobiocin Susceptibility Test: Principle, Procedure, Results”

  1. Ryan McEachern says:

    hello. I am wondering who supplies your novobiocin plates?

    • Tankeshwar Acharya says:

      Thank you for your concern and comment. We prepare the agar plates using commercially available dehydrated media and we purchase Novobiocin disc from the suppliers.

  2. Anonymous says:

    What is the zone of Novobiocin fall between 12-16mn? I found another said >= 16 and <16.

    • Tankeshwar Acharya says:

      Anonymous Ji
      As novobiocin is not included in the CLSI chart, there is slight variation regarding zone size interpretation criteria among manufacture of the disk.
      As this drug is not used for the therapeutic purpose but only for the identification of species of the genera Staphylococcus designating intermediate to an isolate makes no sense. So, if >16 mm is sensitive; then <16 mm has been designated as resistant in some literature. Remember, While performing the test, please follow zone size interpretative chart provided by the manufacture of the disk.

  3. Anonymous says:

    Hello i have question the type and indicator of milk agar ?

  4. Anna Isip says:

    Hi. I complete an experiment but was not sure of my results. , is Novobiocin resistant to s. Epidermis? Is it effective against S. Epidermis ? How about Penicillin and gentamicin are they resistant? Pls explain . I appreciate your help.

  5. says:

    what about the Novobiocin disc concentration for differentation staph saprophyticus and stap epidermidis

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