Infections of the skin and soft tissues are frequently encountered in acute and ambulatory settings and are among the most common infections seen in clinical practice. Data on the exact incidence of skin and soft tissue infections (SSTIs) are lacking because most infections are believed to be mild and treated in outpatient settings. However, SSTIs can range in severity from mild, superficial, self-limiting infections to severe, deep-seated, life-threatening infections.
The skin serves as a barrier between humans and their environment and functions as a primary defense mechanism against infections. The skin consists of the epidermis, the dermis, and subcutaneous fat. The epidermis is the outermost, nonvascular layer of the skin. The dermis is the layer of skin directly beneath the epidermis. It consists of connective tissue and contains blood vessels and lymphatics, sensory nerve endings, sweat and sebaceous glands, hair follicles, and smooth muscle fibers. Beneath the dermis is a layer of loose connective tissue containing primarily fat cells. This subcutaneous fat layer is of variable thickness over the body. Beneath the subcutaneous fat lies the fascia, which separates the skin from muscle. SSTIs may involve any or all layers of the skin, fascia, and muscle. They may also spread far from the initial site of infection and lead to severe complications, such as sepsis, glomerulonephritis, endocarditis, or osteomyelitis.
Intact skin is generally resistant to bacterial, fungal, and viral infections. In addition to providing a mechanical barrier, its relative dryness, slightly acidic pH, colonizing bacteria, frequent desquamation, and sweat prevent invasion by microorganisms.1 Conditions that predispose a patient to SSTIs include: 1) high bacterial load, 2) excessive skin moisture, 3) decreased skin perfusion, 4) damage to corneal layer of skin.2-3 The majority of SSTIs result from disruption of normal host defenses by processes such as skin puncture, abrasion, or underlying disease (eg, diabetes).
Bacterial infections of the skin can be classified as primary or secondary. Primary bacterial infections involve areas of previously healthy skin and are often caused by one or two pathogens. Secondary infections occur in areas of previously damaged skin and are often polymicrobial (Table 22-1). The majority of SSTIs are caused by gram-positive organisms present on the skin surface.4Staphylococcus aureus (S. aureus) and Streptococcus pyogenes (also known as group A streptococci (GAS) and flesh-eating bacteria) account for the majority of SSTIs.
TABLE 22-1 Bacterial Classification of Skin and Soft Tissue Infections |Favorite Table|Download (.pdf)
TABLE 22-1 Bacterial Classification of Skin and Soft Tissue Infections
|Erysipelas||Group A streptococci|
|Impetigo||Staphylococcus aureus, group A streptococci|
|Lymphangitis||Group A streptococci, occasionally S. aureus|
|Cellulitis||Group A streptococci, S. aureus (including caMRSA; occasionally other gram-positive or negative pathogens)|
|Type I||Anaerobes (eg, Clostridium perfringens) Peptostreptococcus, streptococci, Enterobacteriaceae|
|Type II||Group A streptococci|
|Contiguous||S. aureus, streptococci, Enterobacteriaceae, Anaerobes|