Chapter 28 Skin and wound infections
The main resident flora of the skin includes staphylococci – principally Staphylococcus epidermidis (asymptomatic carriage of Staphylococcus aureus is common in specific niches such as the anterior nares and axillae, and in hospital personnel), propionibacteria, micrococci and diphtheroids. Most of them are located superficially in the stratum corneum, but some are found in the hair follicles and act as a reservoir, replenishing the superficial flora after hand-washing. The composition of the normal flora in areas of the body such as the scalp, axillae and pubic area differs considerably because of ecological differences such as the pH, temperature and nutrients (e.g. sebum, fatty acids, urea).
Continuous desquamation of the stratum corneum and the impervious nature of the epithelium are major barriers for invading organisms. Other antimicrobial defences include lysozyme (in sweat, sebum and tears), bacteriocins produced by commensals and fatty acids produced from hydrolysis of sebum triglycerides.
|Aetiological agent||Skin infection|
|Staphylococcus aureus||Abscesses (boils), impetigo, pustules, carbuncles, toxic epidermal necrolysis (Ritter’s disease), omphalitis, angular cheilitis, sycosis barbae|
|β-Haemolytic streptococci||Cellulitis, impetigo, erysipelas|
|Mycobacterium tuberculosis||Lupus vulgaris|
|Mycobacterium ulcerans||Swimming pool granuloma|
|Actinomyces israelii||Actinomycosis (cervicofacial)|
|Herpes simplex virus||Cold sore, herpetic whitlow|
|Varicella-zoster virus||Chickenpox, shingles|
|Coxsackievirus A||Hand, foot and mouth disease|
|Candida spp.||Chronic mucocutaneous candidiasis|
|Various dermatophytes||Ringworm, etc.|
Boils, styes, carbuncles, sycosis barbae and angular cheilitis are all caused by staphylococci. A boil is a common, circumscribed infection of the hair follicle with central suppuration; pus eventually discharges and the boil heals, leaving no scar. Carbuncles, now rare, are large abscesses, which occur at the back of the neck, especially in people with diabetes. They are associated with constitutional upset and malaise. Sycosis barbae is a staphylococcal skin infection involving the shaving area of the face.
Streptococcus pyogenes group A is the most common offender, although Staphylococcus aureus may be involved in some. Cellulitis is a serious disease as subcutaneous spread of infection may carry the pathogen to lymphatic and blood vessels, resulting in marked constitutional upset and septicaemia.
A distinctive type of cellulitis caused by Streptococcus pyogenes is usually seen in the elderly. Lesions are typically on the face and limbs; the lesion distribution on the face is often butterfly-like with a characteristic ‘orange-peel’ texture of the skin and induration; the patient may be acutely ill with high fever and toxaemia.
Necrotizing fasciitis is a rapidly progressing infection involving the full thickness of the skin down to the fascial planes, causing extensive necrosis and tissue loss. The skin looks initially normal, but the infection spreads surreptitiously along the fascial planes, destroying the blood supply to the skin, which discolours and becomes necrotic within hours (hence the tabloid term ‘flesh-eating bacteria’). The patient is severely ill with toxaemia and shock, and may die within 24 h. Formerly called ‘streptococcal gangrene’, it can be caused by a mixed flora comprising staphylococci, strict anaerobes and Enterobacteriaceae; the major offending organism is Streptococcus pyogenes. Management entails prompt excision of skin, antibiotics and supportive therapy.
Inflammation of one or both angles of the mouth, especially in denture-wearing elderly people, may be related to Staphylococcus aureus and/or Candida infection. However, many other predisposing factors are involved, and the dentist should be aware of the management of this condition (see Chapter 35).
Caused by Propionibacterium acnes, acne is a common and disfiguring facial infection of adolescents. The disease is a disorder of the pilosebaceous system and is believed to occur as a result of the production of fatty acids and lipases by bacteria, which initiates an inflammatory response and blocks the ducts, which drain the sebum from the gland to the skin surface. Hormonal imbalances also play a role. Long-term, low-dose antibiotic therapy may alleviate acne in chronic cases.
Caused by Mycobacterium leprae. The organism lives in human skin and nerves and is transmitted by prolonged contact to cause a chronic granulomatous disease. There are two types: the lepromatous and the tuberculoid forms (see Chapter 19).
Gram-negative infections, less frequent than Gram-positive infections, are mostly associated with the moist areas of the skin such as the groin, axillae and perineum. Organisms involved include Pseudomonas aeruginosa and Bacteroides spp.