|
Staphylococcus
ORGANISM:
- Genus: Staphylococcus
- Species: aureus, epidermidis
GENERAL CONCEPTS:
- The staphylococci are divided in two groups based on the presence or absence of the enzyme coagulase. This enzyme converts fibrinogen into fibrin, causing blood plasma to clot. The species called S. aureus is coagulase-positive while S. epidermidis (and other "non-pathogens") is coagulase-negative.
- Typically, staphylococci are opportunistic pathogens or saprophytes.
DISTINCTIVE PROPERTIES:
- Staphylococci are Gram-positive cocci usually arranged in clusters like a bunch of grapes. This appearance is due to the fact that staphylococci divide along two separate planes.
- The morphologically similar streptococci can be differentiated from staphylococci by testing for the enzyme catalase; staphylococci possess this enzyme while streptococci do not.
- Staphylococci possess both group specific and type specific antigens: 90% of S. aureus isolates have protein A. This substance is capable of binding the Fc portion of immunoglobulin IgG. This property helps the bacterium escape the potentially lethal effects of immunoglobulin action and also serves as the basis for some serological tests (coagglutination).
- Toxins produced by S. aureus include: hemolysins, leukocidins, enterotoxin, exfoliative toxin and toxic shock syndrome (TSS) toxin.
- Extracellular enzymes produced by S. aureus include: coagulase, fibrinolysin, DNAse, lipases and hyaluronidase.
PATHOGENESIS:
- Coagulase negative strains of Staphylococcus are generally non-invasive. Under certain conditions, however, they may cause severe disease (e.g. S. epidermidis and subacute endocarditis).
- S. aureus is a common cause of boils, sties and skin infections. Serious (life-threatening) infections (pneumonia, deep abscesses, meningitis) may occur in debilitated persons.
- S. aureus is the most common cause of Gram-positive bacteremia, most commonly involving hospital strains of the organism.
- S. aureus is also responsible for scalded skin syndrome and toxic shock syndrome. It is the most common cause of food poisoning. Symptoms occur only a few hours following ingestion of preformed enterotoxin but large amounts of toxin are required.
HOST DEFENSES:
- Staphylococci have a long association with humans and make up a major portion of our skin flora. Because of this relationship, there are many factors acting both ways. Staphylococci generally resist host defenses. Antibody may help in certain circumstances but protein A prevents opsonization and the action of complement.
EPIDEMIOLOGY:
- These organisms are ubiquitous. Prior to 1950, most staphylococci were sensitive to penicillin; now, most are resistant (hospital strains). Synthetic penicillins have been very useful but now resistance to vancomycin is spreading.
DIAGNOSIS:
- Clinical: Generally, a Gram stain of exudate from a lesion can demonstrate the characteristic Gram-positive cocci arranged in clusters.
- Laboratory: Isolation techniques employ blood agar, mannitol salt agar or potassium-tellurite agar. Bacteriophage testing or serotyping may be utilized.
CONTROL:
- Sanitary: There is virtually no possibility to eliminate these organisms because they are (and have been) a significant part of human normal flora. To control the spread of disease, however, clean hospitals and proper food handling are paramount.
- Immunological: Nothing really available.
- Chemotherapeutic: Antibiotics can be used if life-threatening. One should use penicillin if the particular strain is susceptible. Otherwise, methicillin, oxacillin, cephalosporins or vancomycin may be required. Often, surgical drainage is an important treatment.
|
|
