Staphylococcus aureus

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 grapelike clusters
   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.
2. Culture:
   1. Blood Agar– Colony morphology:  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 commonly used to screen S. aureus: Gives yellow colored 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).