Skin and Soft Tissue Infections

SKIN AND SOFT TISSUE INFECTIONS

Infections of the skin and soft tissue commonly follow traumatic injury to the epithelium but occasionally may be blood-borne. Interruption of the integrity of the skin allows ingress of microorganisms to produce superficial, localized infections which on occasion may become more deep-seated and spread rapidly through tissues. Skin trauma complicates surgical incisions and accidents, including burns. Similarly, prolonged immobilization can result in pressure damage to skin from impaired blood flow. It is most commonly seen in patients who are unconscious.

Microbes responsible for skin infection often arise from the normal skin flora, which includes Staph. aureus. In addition Strep. pyogenes, Ps. aeruginosa and anaerobic bacteria are other recognized pathogens. Viruses also affect the skin and mucosal surfaces, either as a result of generalized infection or localized disease as in the case of herpes simplex. The latter is amenable to antiviral therapy in selected patients, although for the majority of patients, virus infections of the skin are self-limiting.

Strep. pyogenes is responsible for a range of skin infections: impetigo is a superficial infection of the epidermis which is common in childhood and is highly contagious; cellulitis is a more deep-seated infection which spreads rapidly through the tissues to involve the lymphatics and occasionally the bloodstream; erysipelas is a rapidly spreading cellulitis commonly involving the face, which characteristically has a raised leading edge due to lymphatic involvement. Necrotizing fasciitis is a more serious, rapidly progressive infection of the skin and subcutaneous structures including the fascia and musculature. Despite early diagnosis and high-dose intravenous antibiotics, this condition is often life-threatening and may require extensive surgical debridement of devitalized tissue and even limb amputation to ensure survival. A fatal outcome is usually the result of profound toxaemia and bloodstream spread. Penicillin is the drug of choice for all these infections, usually in combination with other agents such as an aminoglycoside and metronidazole in the case of necrotizing fasciitis; in severe instances parenteral administration is appropriate. The use of topical agents, such as tetracycline, to treat impetigo may fail as drug resistance is now recognized.

Staph. aureus is responsible for a variety of skin infections which require therapeutic approaches different from those of streptococcal infections. Staphylococcal cellulitis is indistinguishable clinically from streptococcal cellulitis and responds to flucloxacillin, but generally fails to respond to penicillin owing to penicillinase (β- lactamase) production. In hospital-acquired infection, and occasionally in community practice, MRSA must be considered as a possibility, particularly where the patient is known to be colonized. Staph. aureus is an important cause of superficial, localized skin sepsis which varies from small pustules to boils and occasionally to a more deeply invasive, suppurative skin abscess known as a carbuncle. Antibiotics are generally not indicated for these conditions. Pustules and boils settle with antiseptic soaps or creams and often discharge spontaneously, whereas carbuncles frequently require surgical drainage. Staph. aureus may also cause postoperative wound infections, sometimes associated with retained suture material, and settles once the stitch is removed. Antibiotics are only appropriate in this situation if there is extensive accompanying soft tissue invasion. Rarely, strains of Staph. aureus may express a toxin complex known as Panton-Valentine Leukocidin (PVL); these strains can cause severe sepsis and an often fatal necrotizing pneumonia in young, otherwise fit, patients. The treatment for such infections usually aims to minimize toxin production using protein synthesis inhibitors such as clindamycin plus rifampicin, in combination with linezolid.

Anaerobic bacteria are characteristically associated with foul-smelling wounds. They are found in association with surgical incisions following intra-abdominal procedures and pressure sores, which are usually located over the buttocks and hips where they become infected with faecal flora. These infections are frequently mixed and include Gram-negative enteric bacilli, which may mask the presence of underlying anaerobic bacteria. The principles of treating anaerobic soft tissue infection again emphasize the need for removal of all foreign and devitalized material. Antibiotics such as metronidazole or clindamycin should be considered where tissue invasion has occurred.

The treatment of infected burn wounds presents a number of peculiar facets. Burns are initially sterile, especially when they involve all layers of the skin. However, they rapidly become colonized with bacteria whose growth is supported by the protein-rich exudate. Staphylococci, Strep. pyogenes and, particularly, Ps. aeruginosa frequently colonize burns and may jeopardize survival of skin grafts and occasionally, and more seriously, result in bloodstream invasion. Treatment of invasive Ps. aeruginosa infections requires combined therapy with an aminoglycoside, such as gentamicin or tobramycin, and an anti-pseudomonal agent, such as ceftazidime or piperacillin. This produces high therapeutic concentrations which generally act in a synergistic manner. The use of aminoglycosides in patients with serious burns requires careful monitoring of serum concentrations to ensure that they are therapeutic yet non-toxic, as renal function is often impaired in the days immediately following a serious burn.