Staphylococcus aureus: Disease, Properties, Lab Diagnosis

Last updated on June 25th, 2021

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

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

Main diseases caused by Staphylococcus aureus

  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)

Important Properties of Staphylococcus aureus

Staphylococcus in Gram Stain
Staphylococcus in Gram Stain
  1. 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. 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. 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 not available.

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 is also resistant to methicillin and other classes of antibiotics, limiting the available treatment options.

About Acharya Tankeshwar 476 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.