Chapter 284

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

### Epidemiology

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.

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

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

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 Identification ...

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