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Staphylococci are part of the normal flora and important human
pathogens. There are more than 26 species but only a few are
pathogenic. Staphylococcus aureus is the most invasive species,
which can be differentiated from other species by its possession of
the enzyme coagulase.
Asymptomatic carriage of S. aureus is found in up to 40% of
healthy people, in the nose, skin, axilla or perineum. This is important
in healthcare workers especially if they carry an invasive or
resistant strain (e.g. MRSA).
Staphylococcus aureus has many potential pathogenicity determinants
such as coagulase, which catalyses the conversion of fibrinogen
to fibrin thereby providing protection.
Staphylococcus aureus causes a wide range of infectious
Staphylococcus aureus grows readily on most laboratory media.
Selective medium contains high salt, to which S. aureus is relatively
tolerant. Phenotypic identification depends on demonstrating
coagulase, catalase enzymes and typical 'cluster of grapes' morphology
on Gram stain. Typing by molecular means can support
interventions to control outbreaks.
The history of the susceptibility of S. aureus is a lesson in the
history of antimicrobial chemotherapy.
- It was initially susceptible to penicillin, but strains that produced
�-lactamase soon predominated, so methicillin and
related agents (e.g. flucloxacillin) were introduced and replaced
- Methicillin-resistant S. aureus (MRSA) emerged. Resistance is
caused by possession of the mecA gene, which codes for a penicillin-
binding protein that binds the drug less well. Glycopeptides,
such as vancomycin or teicoplanin, started to be required for these
- Intermediate or heteroresistance to glycopeptides emerged as an
increasing issue and fully glycopeptide-resistant strains (GRSA)
have now been described, resistance being mediated by the vanA
vanB genes acquired from enterococci.
Other antibiotics that remain effective include linezolid,
aminoglycosides, erythromycin, clindamycin, fusidic acid, chloramphenicol
In methicillin-sensitive strains, first- and second-generation
cephalosporins are effective. Fusidic acid may be given with
another agent; it is often given in bone and joint infections (see
Infections of the bones and joints
) because of its tissue penetration.
Prevention and control
Staphylococcus aureus spreads by airborne transmission and via
the hands of healthcare workers. Patients colonized or infected
with MRSA or GRSA should be isolated in a side room with
wound and enteric precautions (see Infection in the hospital environment
). Staff may become
carriers and disseminate the organism widely in the hospital environment.
Carriage may be eradicated by using topical mupirocin
Staphylococcus epidermidis is the most important of the coagulasenegative
staphylococci (CoNS). Once dismissed as contaminants,
these organisms are now recognized as pathogens if conditions
favour their multiplication.
Staphylococcus epidermidis causes infection of intravenous cannulae,
long-standing intravascular prosthetic devices, ventriculoperitoneal
shunts and prosthetic joints, which may lead to
bacteraemia or endocarditis and require the removal of the prosthesis.
Biofilm production contributes to their pathogenicity.
Staphylococcus epidermidis grows readily on laboratory media;
coagulase is not produced. Speciation is by biochemical testing.
DNA restriction patterns or other molecular techniques may be
needed to determine whether strains are identical. S. epidermidis
and other CoNS are common contaminants in blood cultures,
requiring careful evaluation of their clinical significance.
This group of organisms is uniformly susceptible to vancomycin
and usually to teicoplanin. They may also be susceptible to any of
the other agents used for S. aureus infection, but this is unpredictable.
Treatment must be guided by in vitro testing.
Less common than S. epidermidis, Staphylococcus haemolyticus
causes a similar disease pattern. It differs from S. epidermidis in
that it causes haemolysis on blood agar. More importantly, it is
naturally resistant to teicoplanin; significant infections require
These CoNS are a common cause of urinary tract infection in
young women. They can be rapidly distinguished from other
species by their resistance to novobiocin.