Pathogenesis and risk factors

The skin provides an important physical barrier to infection. The normal commensal flora helps to prevent the multiplication and invasion of pathogens. Infections of the skin and deeper tissues often follow trauma, surgery or burns but may arise following even minor damage to the epidermis. Patients who have an impaired immune system (e.g. diabetes mellitus, steroid use, AIDS) are at greater risk of infection.

Some infections involve only the superficial structures of the skin, while others affect the deeper soft tissues below the dermis (Fig. 3.20.1). The infection may be localized or spreading depending on the tissue plane involved and the virulence of the pathogen.

Fig. 3.20.1 Skin structures and infection.

Bacterial super-infections may complicate skin disorders such as eczema. In addition some systemic infections also present with skin features (Fig. 3.20.2).

Fig. 3.20.2 Systemic bacterial infections with skin features. A. Meningococcal septicaemia (Neisseria meningitidis) with petechi. B. Toxic shock syndrome (Staphylococcus aureus) with rash and desquamation. C. Scarlet fever (Streptococcus pyogenes) with an erythematous rash. D. Syphilis (Trepenoma pallidum) with a disseminated rash in second stage (Reproduced with permission from Spicer W J 2008 Clinical microbiology and infectious diseases, 2edn, Churchill Livingstone, Edinburgh).

Bacterial skin infections

Folliculitis is an infection of the hair follicles often caused by Staphylococcus aureus or, if associated with contaminated hot spa pool water, by Pseudomonas aeruginosaFuruncles (boils) consist of walled-off collections of organisms and associated inflammatory cells in follicles and sebaceous glands that eventually ‘point’ and discharge pus. A rare and serious complication is the development of a cluster of boils (carbuncle) occurring on the neck, back or thighs, often in conjunction with fever. Recurrent boils may be associated with skin carriage of S. aureus, in particularly in the nose and requiring treatment with antiseptics or topical antimicrobial drugs.

Impetigo is an infection caused by S. aureus or Streptococcus pyogenes and is limited to the epidermis, presenting as yellow crusting lesions, most often on the face in young children. When the dermis is infected, a red raised demarcated rash appears (erysipelas), often together with fever. Certain strains of S. pyogenes can lead to immune-mediated kidney failure (glomerulonephritis) following infection.

‘Scalded skin syndrome’ is an acute infection of babies and young children caused by a staphylococcal toxin and affecting the epidermis, leading to large areas of skin loss.

Wound infections may develop after a traumatic or surgical wound gets infected with bacteria (e.g. S. aureus). The wound is typically red, swollen and hot to the touch. Necrotic tissue or foreign materials (including sutures) enhance bacterial growth and should be surgically removed (debrided). Localized ‘walled-off’ infection leads to the formation of an abscess, which will need drainage.

Some organisms release gas (e.g. Clostridium spp.) into the tissue (gas gangrene), which may be detected clinically as crepitus or seen in soft tissue radiographs. This typically follows trauma, ischaemia or contaminated surgery such as lower-limb amputation.

Wound infections or even minor skin abrasions may be complicated by the spreading of bacteria (S. aureus, S. pyogenes) beneath the dermis and involving subcutaneous fat (cellulitis). This severe condition may occur at any site but commonly involves the legs and presents with a demarcated red lesion, often with blisters. The patient is usually systemically unwell (febrile and tachycardic). Non-vitalized tissue leads to necrosis (dead tissue) and gangrene. Widespread necrosis of deeper tissues is seen in the life-threatening necrotizing fasciitis caused by toxin-producing ‘flesh-eating bugs’ (e.g. S. pyogenes).

Synergistic gangrene is a polymicrobial infection (multiple different bacteria) that leads to necrosis of the groin and genital soft tissue. Necrotizing or gangrenous conditions require antibiotics and extensive debridement to avoid a fatal outcome.

Bites from humans (and ‘clenched fist injuries’), dogs, cats, other pets and wild animals may lead to local or systemic bacterial infections. Typical causative organisms are staphylococci, streptococci and anaerobes. Infections with Pasteurella multocida is common following dog and cat bites.

Mycobacterial infections of the skin in primary infection with M. tuberculosis (lupus vulgaris) is rare, whereas M. leprae classically causes red anaesthetic lesions on the face, body and limbs in tuberculoid leprosy (Ch. 15). Other skin infections may be caused by atypical mycobacteria, notably the ‘fish-tank’ or ‘swimming pool’ granuloma (M. marinum) or the necrotizing tropical Buruli ulcer (M. ulcerans).