Staphylococcus aureus

Staphylococcus aureus (or Staph aureus) is a type of bacteria commonly found on the skin and hair as well as in the noses, respiratory tract and throats of people and animals. These bacteria are present in up to 25 percent of healthy people and are even more common among those with skin, eye, nose, or throat infections. It affects around 500,000 patients in American hospitals annually.

Brief History

Staphylococcus was first identified in 1880 in Aberdeen, Scotland, by the surgeon Sir Alexander Ogston in pus from a surgical abscess in a knee joint. In 1884, Rosenbach described the two pigmented colony types of staphylococci and proposed the appropriate nomenclature: Staphylococcus aureus (yellow) and Staphylococcus albus (white). 

Scientific classification

Domain:         Bacteria

Kingdom:       Eubacteria

Phylum:         Firmicutes

Class:             Coccus

Order:             Bacillales

Family:           Staphylococcaceae

Genus:           Staphylococcus

Species:         S. aureus

Characteristics

§  Gram-positive.

§  Catalase positive.

§  Approximately 0.5-1.5 µm in diameter.

§  Nonmotile.

§  Non-spore-forming.

§  Facultative anaerobes.

§  Usually found in clusters.

§ 

 Cultural Characteristics

1.    On Nutrient agar-

§  The colonies are large circular convex, smooth, shiny, opaque.

§  They are easily emulsifiable.

§  Most stains produce golden pigmentation.

2.    On Blood agar-

§  Colonies are white to creamy.

§  Most stains produce Beta-haemoloysis. 

3.    On Mannitol salt agar (MSA)-

§  Staphylococcus aureus ferments mannitol and appear as yellow colonies.

§  MSA is a useful selective medium for recovering Staphylococcus aureus from faecal specimens, when investigating food poisoning. 

Role in 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. The spectrum of disease caused by Staphylococcus aureus ranges from mild skin infections to serious systemic disease. Some of these infections 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 and toxic shock syndrome by release of super antigens into the blood stream. 

§  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 septicemia

§  Systemic infections can include any part of the body, but is most commonly seen in pneumonia, bacteremia, wounds and osteomyelitis (infection of the bone).

S. aureus is responsible for many infections, but it may also occur as a commensal. The presence of S. aureus does not always indicate infection. It can survive from hours to weeks, or even months, on dry environmental surfaces, depending on strain.

Duration of illness

§  Some people can be colonized with S. aureus and never get an infection.

o   For those people who do get an infection, the time from exposure to development of disease can be from days to years.

§  Many common skin infections caused by S. aureus will heal without medical treatment.

o   However, some skin infections will require incision and drainage of the infected site and some infections may require antibiotics.

§  Most skin infections will heal within a few weeks.

§  More serious skin infections can take longer to heal if treatment is delayed or if ineffective treatment is given.

§  Some serious S. aureus infections (such as pneumonia or bloodstream infections) typically require hospitalization and treatment with intravenous antibiotics.

Pathogenesis

S. aureus expresses many potential virulence factors:

§  Surface proteins that promote colonization of host tissues;

§  Invasins that promote bacterial spread in tissues (leukocidin, kinases, hyaluronidase);

§  Surface factors that inhibit phagocytic engulfment (capsule, Protein A);

§  Biochemical properties that enhance their survival in phagocytes (carotenoids, catalase production);

§  Immunological disguises (Protein A, coagulase);

§  Membrane-damaging toxins that lyse eukaryotic cell membranes (hemolysins, leukotoxin, leukocidin;

§  Exotoxins that damage host tissues or otherwise provoke symptoms of disease (SEA-G, TSST, ET); and

§  Inherent and acquired resistance to antimicrobial agents. 

For the majority of diseases caused by S. aureus, pathogenesis is multifactorial, so it is difficult to determine precisely the role of any given factor. However, there are correlations between strains isolated from particular diseases and expression of particular virulence determinants, which suggests their role in a particular diseases. The application of molecular biology has led to advances in unraveling the pathogenesis of staphylococcal diseases. Genes encoding potential virulence factors have been cloned and sequenced, and many protein toxins have been purified.

Diagnosis

§  Definitive diagnosis of S. aureus infection is made by obtaining a culture from the area of suspected infection.

§  Suspect diagnosis is based on patient symptoms and the healthcare provider’s evaluation.

Transmission

§  S. aureus is most often spread to others by contaminated hands.

§  The skin and mucous membranes are usually an effective barrier against infection. However, if these barriers are breached (e.g., skin damage due to trauma or mucosal damage due to viral infection) S. aureus may gain access to underlying tissues or the bloodstream and cause infection.

§  Persons who are immunocompromised or who have invasive medical devices are particularly vulnerable to infection.

Treatments

§  Antibiotics commonly prescribed to treat staph infections include certain cephalosporins or related antibiotics, sulfa drugs or vancomycin.

§  Wound drainage

§  Some skin infections do not require treatment.