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Staphylococcus
Staphylococcus is a genus of gram-positive bacteria. Under the microscope
they appear round (cocci), and form in grape-like clusters (staphyl is
Greek for bunch of grapes).
There are many species of staphylococci, most are completely harmless,
and reside normally on the skin.
Staphylococcus aureus
One harmful species is Staphylococcus aureus, which can infect wounds.
Morphology
Staphylococcus aureus is a bacterium that causes illnesses ranging
from minor skin infections and abscesses, to life-threatening diseases
such as pneumonia, meningitis and septicemia.
Cultural characters
More characteristic is its appearance when grown out on agar plates.
It appears as large, round golden-yellow (which is where the name aureus
comes from) colonies, with beta-haemolysis of blood agar. They are facultative
anaerobes.
Antigenic composition
Antigens are one of the mechanisms by which staphylococci can cause
disease (pathogenesis). Determination of antigenic composition also helps
in the classification of staphylococcus strains.
Important cell wall components and antigens include:
- Peptidoglycan
- Teichoic acid
- Protein A
Pathogenesis
Staphylococci can produce disease (pathogenesis) by their ability to
multiply and invade tissues and also through the production of extracellular
enzymes and toxins which include:
Coagulase: An enzyme which produces coagulation of plasma around the
surface of the staphylococcus and fibrin deposition around the lesions
of infection. This protects the staph bacteria from body defense mechanisms
such as phagocyctosis (a process by which the body's immune cells 'eat
up' the bacterium).
Haemolysins: They cause lysis of red blood cells in can be demonstrated
in blood agar.
Exfoliative toxin: The toxin responsible for scalded skin syndrome.
The toxin causes desquamation especially in children.
Toxic shock syndrome toxin: A superantigen that causes toxic shock
syndrome by release of large amounts of cytokines from immun T cells and
macrophages.
Enterotoxins: They are responsible for causing staphylococcal food
poisoning.
Others: Catalase, leucocidine, hyaluronidase, staphylokinase, proteinase
and lipase.
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Diseases caused by staphylococcus
Each year some 500,000 patients in American hospitals contract a staphylococcal
infection. By changing its chemical makeup slightly to evade attack, S.
aureus has become resistant to many commonly used antibiotics. In 1997,
physicians were alarmed to encounter staph strains that resist even vancomycin,
which used to work when all else failed.
Focal suppuration
- Superficial infections (folliculitis, carbuncles, boils, abscess
formation).
- Deep seated infections (osteomyelitis, bronchopneumonia, empyema,
endocarditis, meningitis).
- Bacteremia
- Hospital acquired infections
Toxigenic disease
- Food poisoning
- Toxic shock syndrome
- Scalded skin syndrome
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