Skin and soft tissue infections (SSTIs), also called skin and skin structure infections, are among the most common infections encountered in clinical practice. While most cases present as mild infections, severity of SSTIs can range extensively, from minor and superficial infections that are self-limiting, to severe and deep-seated infections that may be life-threatening.
The skin serves as a primary defense mechanism against infections by acting as a barrier between the hosts and their environment. The skin consists of three layers: the epidermis, dermis, and subcutaneous fat. Skin and soft tissue infections may involve any or all layers of the skin, as well as the fascia and muscle underneath it. In rare instances, SSTIs may spread from the initial site of infection and cause severe complications such as sepsis, bacteremia, osteomyelitis, endocarditis, and glomerulonephritis.
The intact skin is generally impervious to infectious pathogens; however, disruption of this normal host defense (eg, excessive moisture, decreased skin perfusion, or physical damage) predisposes the skin to infections. Most SSTIs originate from a mechanical disruption of the skin, such as a puncture or abrasion, but can also commence as a complication of an underlying disease, such as diabetes mellitus.
Two categories of SSTIs exist: primary and secondary. In primary infections, healthy skin is infected, typically by a single microorganism. Secondary infections occur in pre-existing damaged skin and are often polymicrobial. Table 24-1 lists the common etiologic pathogens responsible for specific types of SSTIs. Gram-positive organisms present on skin, particularly Staphylococcus aureus (including community-associated methicillin-resistant S. aureus [caMRSA]) and Streptococcus pyogenes (also known as group A streptococci [GAS] or β-hemolytic streptococci) cause the majority of SSTIs. In the following sections, different types of SSTIs and their treatment strategies are presented.
TABLE 24-1Bacterial Classification of Skin and Soft Tissue Infections ||Download (.pdf) TABLE 24-1 Bacterial Classification of Skin and Soft Tissue Infections
|Primary Infections ||Pathogen |
|Folliculitis, Furuncles, Carbuncles ||S. aureus (including caMRSA), P. aeruginosa, Candida species |
|Impetigo ||S. aureus, group A streptococci |
|Lymphangitis ||Group A streptococci, occasionally S. aureus |
|Erysipelas ||Group A streptococci |
|Cellulitis ||Group A streptococci, S. aureus (including caMRSA), occasionally other gram-positive, gram-negative, or anaerobic pathogens |
|Necrotizing fasciitis || |
| Type I ||Anaerobes (eg, Bacteroides spp., Peptostreptococcus spp.), streptococci, Enterobacteriaceae |
| Type II ||Group A streptococci, caMRSA |
|Secondary Infections |
|Diabetic foot infections ||S. aureus, streptococci, Enterobacteriaceae, anaerobes, P. aeruginosa |
|Pressure sores ||S. aureus, streptococci, Enterobacteriaceae, anaerobes, P. aeruginosa |
|Bite wounds || |
| Animal (dog, cat) ||Pasteurella multocida, S. aureus, streptococci, Bacteroides spp., oral anaerobes |
| Human ||Eikenella corrodens, S. aureus, streptococci, Corynebacterium spp., Bacteroides spp., Peptostreptococcus, oral anaerobes |
|Burn wounds ||P. aeruginosa, Enterobacteriaceae, Acinetobacter species, S. aureus, streptococci |
|Hematogenous ||S. aureus |
|Contiguous ||S. aureus, streptococci, Enterobacteriaceae, anaerobes |
|Vascular insufficiency ||S. aureus, streptococci, Enterobacteriaceae, anaerobes |