Staphylococcus Aureus and Disease

Staphylococcus aureus (S. aureus) is one of the leading causes of infections acquired in the community and after surgery or hospital. Around 30% of individuals carry S. aureus in their nose, pharynx or back of throat and on their skin.

S. aureus causes numerous infections at various sites of the body. Some of these include:

• Skin infections – S. aureus causes boils, furuncles, styes, impetigo and other superficial skin infections in humans
• Infections of surgical and trauma wounds – Those with chronic illness, diabetes, traumatic injury, burns or immunosuppression are susceptible to more severe skin, deeper tissue infections and deep abscesses
• Urinary tract infections
• Food poisoning and gastrointestinal tract infections may be caused by consuming food contaminated with S. aureus.
• Infections of organs include pneumonia (lung infection), osteomyelitis (bone infection), endocarditis (heart infection), phlebitis (infection of veins and blood vessels), mastitis (infection of breast and formation of abscesses) and meningitis (brain infections). These infections are more common in hospitalized patients rather than healthy individuals in the community.
• Infections from and on indwelling medical devices. These include infection of joint prostheses, cardiovascular devices and artificial heart valves.
• Generalized life threatening blood infections or Toxic shock syndrome (TSS), bacteremia and septicaemia

Pathogenesis

S. aureus expresses quite a few extracellular proteins that are virulent to the host. For the majority of diseases caused by this organism, pathogenesis is multifactorial.

In order to initiate infection the bacteria needs to gain access to the host and attach to host cells or tissues. S. aureus has numerous surface proteins that promote attachment to host proteins such as laminin and fibronectin that form part of the extracellular matrix. Fibronectin is also present on epithelial and endothelial surfaces and is also a part of blood clots. The bacteria have a fibrinogen/fibrin binding protein that help it to attach to blood clots and traumatized tissue. This is the reason why S. aureus is capable of producing wound infections and post-surgery infections.

S. aureus also has numerous factors that help it to evade the host defence mechanisms. For example, many of the strains carry a polysaccharide on their surface which may help them to resist phagocytosis by macrophages. Protein A is a surface protein of S aureus which binds immunoglobulin G molecules by the Fc region. This disrupts opsonization and phagocytosis.

S. aureus can cause severe damage to the host. It makes several types of protein toxins which are probably responsible for symptoms during infections. These toxins damage the membrane of the red blood cells and lead to their breakdown. They also produce leukocidin that causes membrane damage to leukocytes. Systemic release of α-toxin causes septic shock, while enterotoxins and TSST-1 cause toxic shock.