Bacterial infections of the skin can be classified as primary or secondary (Table 47–1). Primary bacterial infections are usually caused by a single bacterial species and involve areas of generally healthy skin (eg, impetigo and erysipelas). Secondary infections, however, develop in areas of previously damaged skin and are frequently polymicrobic.
The conditions that may predispose a patient to the development of skin and soft-tissue infections (SSTIs) include (1) a high concentration of bacteria; (2) excessive moisture of the skin; (3) inadequate blood supply; (4) availability of bacterial nutrients; and (5) damage to the corneal layer, allowing for bacterial penetration.
The majority of SSTIs are caused by gram-positive organisms and, less commonly, gram-negative bacteria present on the skin surface. Staphylococcus aureus and Streptococcus pyogenes account for the majority of SSTIs. Community-associated methicillin-resistant S. aureus (CA-MRSA) has emerged and is often isolated in otherwise healthy patients.
TABLE 47–1Bacterial Classification of Important Skin and Soft-Tissue Infections |Favorite Table|Download (.pdf) TABLE 47–1 Bacterial Classification of Important Skin and Soft-Tissue Infections
|Primary infections |
|Erysipelas ||Group A streptococci (Streptococcus pyogenes) |
|Impetigo ||Staphylococcus aureus (including methicillin-resistant strains), group A streptococci |
|Lymphangitis ||Group A streptococci; occasionally S. aureus |
|Cellulitis ||Group A streptococci, S. aureus (potentially including methicillin-resistant strains); occasionally other gram-positive cocci, gram-negative bacilli, and/or anaerobes |
|Necrotizing fasciitis |
|Type I ||Anaerobes (Bacteroides spp., Peptostreptococcus spp.) and facultative bacteria (streptococci, Enterobacteriaceae) |
|Type II ||Group A streptococci |
|Type III ||Clostridium perfringens |
|Secondary infections |
|Diabetic foot infections ||S. aureus, streptococci, Enterobacteriaceae, Bacteroides spp., Peptostreptococcus spp., Pseudomonas aeruginosa |
|Pressure sores ||S. aureus including methicillin-resistant strains, streptococci, Enterobacteriaceae, Bacteroides spp., Peptostreptococcus spp., P. aeruginosa |
|Bite wounds |
|Animal ||Pasteurella spp., S. aureus, streptococci, Bacteroides spp. |
|Human ||Eikenella corrodens, S. aureus, streptococci, Corynebacterium spp., Bacteroides spp., Peptostreptococcus spp. |
|Burn wounds ||P. aeruginosa, Enterobacteriaceae, S. aureus, streptococci |
Erysipelas (Saint Anthony’s fire) is an infection of the superficial layers of the skin and cutaneous lymphatics. The infection is almost always caused by β-hemolytic streptococci, with S. pyogenes (group A streptococci) responsible for most infections.
The lower extremities are the most common sites for erysipelas. Patients often experience flu-like symptoms (fever and malaise) prior to the appearance of the lesions. The infected area is painful, often a burning pain. Erysipelas lesions are bright red and edematous with lymphatic streaking and clearly demarcated raised margins. Leukocytosis is common, and C-reactive protein is generally elevated.
Mild to moderate cases of erysipelas in adults are treated with intramuscular procaine penicillin G or penicillin VK. For more serious infections, aqueous penicillin G, 2 million to 8 million units daily, should be administered intravenously (IV). Penicillin-allergic patients can be treated with clindamycin or erythromycin.
Evidence-based recommendations for treatment of SSTIs are found in Table 47–2, and recommended drugs and dosing regimens for outpatient treatment of mild to moderate SSTIs are found ...
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