Staphylococcus aureus: Disease, Properties, Lab Diagnosis

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

Major Characteristics of Staphylococcus aureus

  1. Gram stain: Staphylococci appear as Gram-positive cocci that occur singly and in pairs, tetrads, short chains, and irregular grape-like clusters
  2. Catalase Test: Positive
  3. Coagulase Test:  Positive
  4. Non-motile
  5. Non-sporing
  6. Often unencapsulated or have a limited capsule
  7. Facultative anaerobes.

Main diseases caused by Staphylococcus aureus

Mneomonic: Diseases caused by Staphylococcus can be remembered using this acronym “SOFTPAINS”

  1. Skin Infections & Surgical wound infections
  2. Osteomyelitis
  3. Food poisoning/gastroenteritis
  4. Toxic shock syndrome
  5. Pneumonia (mainly hospital-acquired)
  6. Acute endocarditis
  7. Infective arthritis
  8. Necrotizing fasciitis
  9. Sepsis and Staphylococcal scalded skin syndrome (SSSS)
Staphylococcus in Gram Stain
Staphylococcus in Gram Stain

Virulence factors of S. aureus

  1. Surface proteins: Protein A (prevents activation of complement), clumping factor, teichoic acid (adherence and induction of septic shock)
  2. Superantigens: Enterotoxin A-D, Exfoliatin A&B, Toxic Shock Syndrome Toxin (TSST)-superantigen.
  3. Cytotoxins: α-hemolysin, β-hemolysin,γ-hemolysin, δ-hemolysin, Panton-Valentine Leukocidin (PVL), Exfliatin (epidermolytic-cleaves desmoglein in desmosomes)
  4. Enzymes: Nuclease, lipase, hyaluronidase, coagulase  (causes plasma to clot), staphylokinase, glycerol ester hydrolase, catalase (degradation of H2O2 limits the ability of neutrophils to kill S. aurues), etc.

Laboratory diagnosis

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)
  1. Gram staining: Gram-positive cocci in clusters, cocci may appear singly in pairs or in short chains.
  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. 
    3. After inoculation, MSA plates were incubated at 35°C for 24 to 48 hours. S. aureus is 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).

Biochemical tests for the identification of S. aureus

Name of the testStaphylococcus aureusNotes
Catalase testPositive To differentiate staphylococci from streptococci.
Hemolysisβ-hemolysis or non-hemolysis 
Coagulase testPositiveTo differentiate S. aureus from CONS.
Mannitol fermentationYesTo differentiate S. aureus (fermenter) from CONS (non-fermenter)
Furazolidone disk TestSensitiveTo differentiate staphylococci from micrococci (resistant)
Polymyxin B sensitivity testResistantMost staphylococcal species are susceptible to polymyxin B, but S. aureus, S. lugdunensis, and S. epidermidis are resistant.
Bacitracin( 0.04-U disk) susceptibility testResistantTo separate staphylococci from micrococci (susceptible)
Microdase testNegativeTo differentiate staphylococci from micrococci.
DNase testPositive To differentiate S.aureus from other Staphylococci (-ve) when coagulase test is unavailable.

Antimicrobial Resistance 

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


  1. Forbes, S., Sahm, D. F., & Weissfeld, A. S. (2002). Bailey & Scott’s Diagnostic Microbiology. Mosby.
  2. Foster, T. (1996). Staphylococcus. In S. Baron (Ed.), Medical Microbiology. (4th ed.). University of Texas Medical Branch at Galveston.
  3. Tong, S. Y., Davis, J. S., Eichenberger, E., Holland, T. L., & Fowler, V. G., Jr (2015). Staphylococcus aureus infections: epidemiology, pathophysiology, clinical manifestations, and management. Clinical microbiology reviews, 28(3), 603–661.

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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