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Cellulitis is generally an acute, spreading infectious process that initially affects the epidermis and dermis and may subsequently spread within the superficial fascia.(1) Infections of the skin and soft tissue are among those most commonly seen both in and out of the hospital setting. Allowed to spread, these infections can lead to complications such as gram-negative sepsis, bacterial endocarditis, or streptococcal glomerulonephritis.
The skin and subcutaneous tissues are normally extremely resistant to infection. Even when high concentrations of bacteria are applied topically or injected into the soft tissue, resultant infections are rare.(2)
The effectiveness of skin as a barrier against infection is due, in part, to several factors that work together. The surface of the skin is relatively dry and not conducive to bacterial growth. Also, continuous renewal of the epidermal layer causes shedding of keratocytes and skin bacteria. The skin also produces sebaceous secretions, which hydrolyze to free fatty acids and strongly inhibit the growth of many bacteria and fungi.
Conditions that compromise these host defenses may predispose patients to the development of skin infections and include skin punctures, abrasions, or underlying disease such as diabetes. Large portions of these infections are caused by normal skin flora, and the nature and severity of the infection is dependent upon the site of inoculation and type of microorganism. Exposed areas of the body (face, neck) generally have the highest bacterial density and Staphylococcus epidermidis is the most common microorganism. Moister areas (axilla and groin) are more frequently colonized with gram-negative bacilli.(3)
Cellulitis is classically caused by group A b-hemolytic streptococci such as Streptococcus pyogenes or Staphylococcus aureus. Although less common, cellulitis may also be caused by a wide variety of gram-negative organisms, such as Escherichia coli, Proteus sp., and Klebsiella sp. Cellulitis, as a result of gram-negative organisms, is often polymicrobic in nature and may involve anaerobic microorganisms, especially Bacteroides, and Peptostreptococcus.(4) This type of complicated infection is often seen in patients with diabetic foot problems. It has been reported that approximately 25 percent of the diabetic population experience soft tissue infection at some time during their illness. Infection in the lower extremities is also the most common septic problem leading to hospitalization in diabetics. One of the more exotic possibilities is induction of cellulites by an insect bite. One example of this would be from the bite of the brown recluse spider.
Neuropathic changes to the autonomic nervous system as a consequence of diabetes may affect the motor nerve supply of small intrinsic muscles of the foot, resulting in muscular imbalance, abnormal stresses on tissue and bone, and repetitive injuries.(5) Diminished sensory perception causes an absence of pain and unawareness of minor injuries and ulceration. Also, the sympathetic nerve supply may be damaged and can result in an absence of sweating. This leads to dry, cracked skin, which can become secondarily infected.(6)
Another population predisposed to skin infections and cellulitis is the intravenous drug user. Infectious complications may include, among other things, abscess formation and cellulitis at the site of injection. These skin and soft-tissue infections are frequently located on the upper extremities and are often polymicrobic in nature.
Staphylococcus aureus or streptococci are the most common pathogenic organisms isolated from these infections (37-61 percent of patients). Anaerobic bacteria are also commonly found (6-67 percent of patients), although the role of these bacteria in the pathogenesis of infection
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