Staphytlococcus aureus (S. aureus) is the most virulent Staphylococcus species and causes a variety of infections in humans, both localized and systemic. Most commonly, S. aureus causes a localized skin infection after a skin break or wound. Another common site of entry is the respiratory system, and Staphylococcal pneumonia is a frequent complication of influenza. More serious illnesses may result whn S. aureus enters the blood stream including osteomyelitis, endocarditis, sepsis and meninitis, among others. S. aurues is a major cause of nosomial (hosptial aqcquired) infections. 

S. aureus produces several protenal virulence factors such as alpha, beta, gamma and delta toxins, toxic shock syndrome toxin, enterotoxins, leucocidin, proteases, coagulase and clumping factor. The emergence of antibiotic-resistant forms of S. aureus (e.g., MRSA) is a major problem.

The surface of most strains is coated with protein A that binds the Fc region of IgG. Teichoic acids are bound to both peptidoglycan layer and cytoplasmic membrane. 

Biochemical, Morphological and Structural Characteristics

S. Aureus are alpha hemolytic and destroy red blood cells. They are facultative anaerobic, coccal bacteria. They appear as grape-like clusters (because they reproduce asexually by binary fission, and the two daughter cells remain attached) and have large, golden-yellow colonies, often with hemolysis when grown on blood agar plates. 

Protein A of Staphylococcus aureus (“SpA): refers to a 42 kda multi-domain protein isolated from S. aureus. SpA is bound to the bacterial cell wall via its carboxy-terminal cell wall binding region, referred to as the X domain. At the amino-terminal region, it includes 5 immunoglobulin binding domains, referred to as E, D,A,B and C. Each of these domains contains about 58 amino acid residues and they share 675-90% amino acid sequence identity. SpA is widely used for antibody purification due to its high affinity for immunoglobulins. For more on SpA as an affinity ligand for purification of antibodies see “Protein Purification”. 

Pathogenesis

S. aureus oes much of its pathogenic process to proteins such as Surface protein A (SpA) anchored to its cell well. These adhesins facilitate evasion of th host immune response and host colonisation by binding to plasma proteins and host endothelial cells. SpA is anchored to the bacterial cell surface via an LPXTG motif, which is typical of microbial surface components that recognize adhsive matrix molecules. The binding activity of SpA acts to cloak the bacerial cell with IgG, thus blocking any interaction with Fc receptors on nuetrophils and hingering phagocytosis (Atkins, Molecular Immunology 45, 2008, 1600-1611). 

S. aureus causes disease by either production of toxin or direct invasion and destruction of tissue. Some toxins produced include exfoliative toxin which causes staphylococcal scalded skin syndrome (SSSS), toxic shock syndrome (TSS) and enterotoxins which are heat resistant toxins that cause food poisoning. 

Entry/Colonisation

Adehsion to tissues is required for bacterial colnisation to occur. For this purpose, S aureus express surface adhesins which interact with host matrix proteins such as fibronectin, collagen, etc. In addition, Staphylococci are able to bind several serum proteins, such as IgG, possibly masking the bacteria from the immune system of the host. 

The most studied receptor in S aureus is protein A, a cell wall associated protein, which binds to the Fc and the Fab regions of IgG from several species. The fact that protein A binds so well has been taken advantage of for the purification of IgG (see “biotechnology”, “protein purification” and “affinity A” under “chromatography”). 

Treatment

S aureus are usually treatment with penicillins or cephalosporins. Methicillin resistant S. aureus are resistant to all beta-lactams, however. Only vancomycin is effective.(a characteristic that can be used to distinguish them from S. epidermidis which are gamma hemolytic)