Key Clinical Questions
How does the presentation of skin and soft tissue infection help with diagnostic and therapeutic decisions?
What are the common bacterial causes of cellulitis?
What are the differences in bacterial etiology and therapeutic management between nonpurulent and purulent cellulitis?
What features suggest more complicated infection?
What are the strategies for patients with recurrent infections?
Skin and soft tissue infections are distinguished by their pattern of presentation, tissues involved, microbiology, and response to therapy. Cellulitis is defined as inflammation involving the dermis and subcutaneous tissues. Cellulitis usually is due to an acute bacterial infection. Bacterial cellulitis may occur as rapidly spreading skin erythema without a purulent collection or may be associated with an abscess or infected wound. Occasionally, cellulitis is due to nonbacterial pathogens or noninfectious inflammatory conditions. An abscess is a contained dermal collection of pus. Necrotising fasciitis is rapidly progressive infection with destruction of subcutaneous soft tissue, muscle, and deep fascia. Beta-hemolytic streptococci cause the majority of nonpurulent cellulitis; Staphyloccocus aureus is responsible for most cellulitis cases with abscess formation or arising from infected wounds. Necrotizing fasciitis is commonly due to group A streptococci, or when occurring near the perineum, a mixture of bowel flora.
A breach in skin integrity is usually required for the development of soft tissue infection. The skin injury may be obvious, or too subtle to detect. Tinea pedis is a common portal of entry for bacteria in nonpurulent lower-extremity cellulitis. Other local risk factors for cellulitis include lymphedema, inflammation and edema associated with venous insufficiency, and trauma, including skin injury from injection drug use and bites. Medical risk factors include diabetes, arterial insufficiency, cirrhosis, renal insufficiency, and neutropenia.
Less commonly, soft tissue infection arises from systemic bacteremia, with secondary seeding of the skin. In neutropenic patients, bacterial proliferation in the vessel wall leads to tissue ischemia and skin necrosis. Clinically, this presents as ecthyma gangrenosum, an area of inflamed skin with a hemorrhagic pustule that develops into a necrotic ulcer. In patients with cirrhosis, ingestion of shellfish contaminated with Vibrio species may lead to gastroenteritis, followed by bacteremia, and finally metastatic skin infection with hemorrhagic bullae.
PRESENTATION PATTERNS OF SOFT TISSUE INFECTION
A number of epidemiologic features in cellulitis put patients at risk for particular pathogens. These are summarized in Table 196-1 and discussed in more detail later in this chapter.
TABLE 196-1Cellulitis Pathogens Associated with Particular Epidemiology* ||Download (.pdf) TABLE 196-1 Cellulitis Pathogens Associated with Particular Epidemiology*
|Exposure ||Pathogen |
|Dog bite ||Pasteurella multocida, Capnocytophaga spp., mixed aerobic and anaerobic flora |
|Cat bite ||Pasteurella multocida, mixed aerobic and anaerobic flora |
|Fish exposure ||Mycobacterium marinum, Erysipelothrix rhusiopathiae, Streptococcus iniae |
|Salt water ||Vibrio vulnificus |
|Fresh water ||Aeromonas hydrophilia, Edwardsiella tarda, Chromobacterium violaceum...|