Staphylococcus aureus:Disease, Properties, Pathogenesis, and Laboratory diagnosis

Staphylococcus aureus, a frequent colonizer of the skin and mucosa of humans and animals, is a highly successful opportunistic pathogen.

Main diseases caused by Staphylococcus aureus

  1. Abscesses
  2. Endocarditis
  3. Gastroenteritis (food poisoning)
  4. Toxic shock syndrome
  5. Hospital acquired pneumonia
  6. Sepsis
  7. Surgical wound infections
  8. Staphylococcal scalded skin syndrome (SSSS)

Important Properties of Staphylococcus aureus

  1. Gram positive cocci that occur singly and in pairs, tetrads, short chains, and irregular grape like clusters

    Staphylococcus in Gram Stain
    Staphylococcus in Gram Stain
  2. Catalase Test: positive
  3. Coagulase Test:  positive
  4. Other properties: Non motile, non sporing, often unencapsulated or have a limited capsule, facultative anaerobes.

Virulence factors:

  1. Surface proteins: Protein A (prevents activation of Complement), Clumping factor, Teichoic acid (adherence and induction of septic shock)
  2. Super antigens: Enterotoxin A-D, Exfoliatin A&B, Toxic Shock Syndrome Toxin (TSST)-super antigen.
  3. Cytotoxins: α-hemolysin, β-hemolysin,γ-hemolysin, δ-hemolysin, Panton Valentine Leukocidin (PVL), Exfliatin (epidermolytic-cleaves desmglein in desmosomes)
  4. Enzymes: Nuclease, Lipase, Hyaluronidase, Coagulase  (causes plasma to clot), Staphylokinase, Glyerol ester hydrolase, Catalase (degradation of H2O2 limits the ability of neutrophils to kill S. aurues), etc.

Laboratory diagnosis

  1. Gram staining: Gram positive cocci in clusters is the characteristics, cocci may appear singly in pairs or in short chains.

    Yellow colonies of S. aureus in Mannitol Salt Agar (Photo by Anne Hanson and Matthew Pietraszewski, University of Maine
    Yellow colonies of S. aureus in Mannitol Salt Agar (Photo by Anne Hanson and Matthew Pietraszewski, University of Maine)
  2. Culture:
    1. Blood Agar:  growth occurs abundantly within 18 to 24 hours, yellow or golden yellow colonies with or without Beta hemolysis are seen.
    2. Mannitol Salt Agar (MSA)  is a selective media  commonly used for the isolation of S. aureus. 
      After inoculation, MSA plates were incubated at 35°C for 24 to 48 hours. S. aureus are Mannitol fermenting bacteria and gives yellow or gold colonies.
  3. Biochemical tests: 
  1. Catalase test: Positive
  2. Coagulase test: Positive- Distinguish S. aureus from coagulase negative Staphylococcus aureus (CONS).  CONS are further differentiated on the basis of Novobiocin sensitivity test ( S. epidermidis is sensitive, whereas S. saprophyticus is resistant).

Antimicrobial Resistance 

Staphylococcus aureus, including Methicillin-resistant Staphylococcus aureus (MRSA), are one of the most common causes of healthcare-associated infections. The first report of Vancomycin Resistant Staphylococcus aureus (VRSA)  came in 2002.  VRSA  are also resistant to methicillin and other classes of antibiotics, limiting the available treatment options.

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