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Staphylococci are ubiquitous inhabitants of the skin and mucous
membranes of humans and other mammals. They exist in a commensal
relationship until a breach in a cutaneous or mucosal barrier permits
staphylococcal access to deeper tissues and the bloodstream or until a
foreign body or medical device provides a foothold. The production
of coagulase, an enzyme that clots plasma, distinguishes Staphylococcus
aureus from other medically important staphylococci. Those
that do not produce coagulase are grouped collectively as coagulase-negative
staphylococci (CoNS) and represent the most common resident bacteria of
humans.1 All staphylococci are nonmotile, nonspore-forming,
facultative anaerobic bacteria. In Gram-stained specimens, they
appear as gram-positive cocci in clusters, as well as in pairs and
tetrads. Peptidoglycans and lipoteichoic acids form the basic cell
wall structures of staphylococci and most exhibit microcapsule formation.
Colony morphologies followed up with biochemical reactions allow identification
of pathogenic staphylococci. Typical 24-hour S. aureus colonies
are larger, yellow pigmented, and surrounded by a small zone of
hemolysis. Colonies of S epidermidis are typically
small, white or beige, and approximately 1 to 2 mm in diameter after
overnight incubation. Small colony variants (SCV) of S aureus,
important in some persistent infections, may be missed initially
because of their pinpoint size. Staphylococcal colonies in general
will be catalase positive, distinguishing them from streptococci.1
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Coagulase-negative staphylococci (CoNS) colonize virtually all
normal skin. Because of the commensal nature of the CoNS, they are
often recovered from specimens from superficial sites and may be
recovered from body fluids and deep sites when inadequate or improper collection
techniques have been employed. Recovery of CoNS from a normally
sterile body site must be interpreted in light of the clinical circumstances
of the patient. Of the more than 30 species, at least 15 are indigenous
to humans, with S epidermidis being the most common
of the resident CoNS.
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Staphylococcus hominis and S saccharolyticus are
common resident flora as well, with S haemolyticus and S
warneri less frequent. Other transient colonizers include S
xylosus, S simulans, S cohin,
and S lugdunensis. Selected species are also recognized
for the special niches they colonize, including S capitus (scalp), S
auricularis (ear), and S saprophyticus (genitourinary
tract).2,3
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Although S epidermidis appears to be responsible
for the greatest number of infections, many of the other species
that inhabit humans are recognized to play a pathogenic role in
disease, especially in the health care setting and in the context
of foreign bodies.1,4-6
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S lugdunensis has been recognized for its propensity
to cause severe infections of skin and soft tissuein
the absence of underlying risk factors.5 In one
series, isolates were associated particularly with abscesses in
the perineal and gluteal areas.7,8S lugdunensis endocarditis
has a predilection for native valves and may result in a fulminant
course similar to that of S aureus.9-12...