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
- 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)
Important Properties of Staphylococcus aureus
- Gram positive cocci that occur singly and in pairs, tetrads, short chains, and irregular grape like clusters
- Catalase Test: positive
- Coagulase Test: positive
- Other properties: Non motile, non sporing, often unencapsulated or have a limited capsule, facultative anaerobes.
- Surface proteins: Protein A (prevents activation of Complement), Clumping factor, Teichoic acid (adherence and induction of septic shock)
- Super antigens: Enterotoxin A-D, Exfoliatin A&B, Toxic Shock Syndrome Toxin (TSST)-super antigen.
- Cytotoxins: α-hemolysin, β-hemolysin,γ-hemolysin, δ-hemolysin, Panton Valentine Leukocidin (PVL), Exfliatin (epidermolytic-cleaves desmglein in desmosomes)
- 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.
- Gram staining: Gram positive cocci in clusters is the characteristics, 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 are Mannitol fermenting bacteria and gives yellow or gold colonies.
- Biochemical tests:
- Catalase test: Positive
- 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).
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.