Staphylococcus aureus, a frequent colonizer of the skin and mucosa of humans and animals, is a highly successful opportunistic pathogen.
Major Characteristics of Staphylococcus aureus
- Gram stain: Staphylococci appear as Gram-positive cocci that occur singly and in pairs, tetrads, short chains, and irregular grape-like clusters
- Catalase Test: Positive
- Coagulase Test: Positive
- Often unencapsulated or have a limited capsule
- Facultative anaerobes.
Main diseases caused by Staphylococcus aureus
Mneomonic: Diseases caused by Staphylococcus can be remembered using this acronym “SOFTPAINS”
- Skin Infections & Surgical wound infections
- Food poisoning/gastroenteritis
- Toxic shock syndrome
- Pneumonia (mainly hospital-acquired)
- Acute endocarditis
- Infective arthritis
- Necrotizing fasciitis
- Sepsis and Staphylococcal scalded skin syndrome (SSSS)
- Surface proteins: Protein A (prevents activation of complement), clumping factor, teichoic acid (adherence and induction of septic shock)
- Superantigens: Enterotoxin A-D, Exfoliatin A&B, Toxic Shock Syndrome Toxin (TSST)-superantigen.
- Cytotoxins: α-hemolysin, β-hemolysin,γ-hemolysin, δ-hemolysin, Panton-Valentine Leukocidin (PVL), Exfliatin (epidermolytic-cleaves desmoglein in desmosomes)
- 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.
- Gram staining: Gram-positive cocci in clusters, cocci may appear singly in pairs or in short chains.
- Blood Agar: growth occurs abundantly within 18 to 24 hours, yellow or golden yellow colonies with or without Beta hemolysis are seen.
- 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 is mannitol fermenting bacteria and gives yellow or gold colonies.
- Biochemical tests:
Biochemical tests for the identification of S. aureus
|Name of the test||Staphylococcus aureus||Notes|
|Catalase test||Positive||To differentiate staphylococci from streptococci.|
|Hemolysis||β-hemolysis or non-hemolysis|
|Coagulase test||Positive||To differentiate S. aureus from CONS.|
|Mannitol fermentation||Yes||To differentiate S. aureus (fermenter) from CONS (non-fermenter)|
|Furazolidone disk Test||Sensitive||To differentiate staphylococci from micrococci (resistant)|
|Polymyxin B sensitivity test||Resistant||Most staphylococcal species are susceptible to polymyxin B, but S. aureus, S. lugdunensis, and S. epidermidis are resistant.|
|Bacitracin( 0.04-U disk) susceptibility test||Resistant||To separate staphylococci from micrococci (susceptible)|
|Microdase test||Negative||To differentiate staphylococci from micrococci.|
|DNase test||Positive||To differentiate S.aureus from other Staphylococci (-ve) when coagulase test is unavailable.|
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.