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Normal human skin is colonized soon after birth by a large number of bacteria that live as commensals on the epidermis and epidermal appendages (the skin microbiome). Coagulase-negative Staphylococci (Staphylococcus epidermidis) are inoculated during vaginal passage and coryneform bacteria take up residence on neonatal skin shortly after birth. Within several weeks after birth, the microbiome of neonatal skin is similar to that of adults and includes many species of bacteria and fungi (see Chapter 175).
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The majority of the primary and secondary pyodermas (cutaneous bacterial infections) are caused by either S. aureus or group A Streptococcus. These bacteria cause a broad clinical spectrum of infection ranging from superficial pyodermas to invasive soft-tissue infections (STIs; see Chapter 179) depending on the organism, the anatomic location of infections, and on host factors.
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Staphylococci are classified into two major groups: (1) the coagulase-negative Staphylococci and (2) coagulase-positive (S. aureus) Staphylococci. Individuals carry a minimum of 10–24 combined temporary and resident strains of S. epidermidis, the most common coagulase-negative strain. S. epidermidis is a common colonizer of the skin but is capable of causing superficial and invasive infections (particularly about implants and catheters).
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S. aureus permanently colonizes the anterior nares in approximately 20% of the population. Carriage is transient or intermittent in other individuals. Approximately 60% of healthy individuals have occasional carriage of S. aureus at some site.1 Other sites of colonization include the axillae, perineum, pharynx, and hands. Conditions predisposing to S. aureus colonization include atopic dermatitis, diabetes mellitus (insulin dependent), dialysis (hemo- and peritoneal), intravenous drug use, liver dysfunction, and human immunodeficiency virus (HIV) infection. Colonization by S. aureus is found at some body site in up to 37% of patients presenting with purulent community associated methicillin resistant S. aureus (MRSA) infections.2
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S. aureus is an aggressive pathogen and the most common cause of primary pyodermas and STIs, as well as of secondary infections on disease-altered skin. S. aureus in pyodermas or STIs can invade the bloodstream, producing bacteremia, metastatic infection such as osteomyelitis, and acute infective endocarditis. Some strains of S. aureus also produce exotoxins, which can cause constellations of cutaneous and systemic symptoms such as ...