logo Bacterial infections

Skin infections

Bacterial infections

Objectives

  • Be able to identify and manage common staphylococcal and streptococcal skin infections.
  • Be familiar with the range of appearance of other bacterial infections.

Key points

  • Impetigo is due to superficial skin infection with Staphylococcus aureus and/or Streptococcus pyogenes.
  • Impetigo may complicate wound healing, infestations and dermatitis.
  • Furunculosis (boils) is due to invasion of the hair follicle by Staphylococcus aureus.
  • Staphylococcal skin infections may rarely be complicated by widespread rashes including toxic shock syndrome and staphylococcal scalded skin syndrome.
  • Streptococcal skin infections may rarely be complicated by widespread rashes including scarlet fever and toxic shock-like syndrome.
  • If staphylococcal infections recur, treat carrier sites (nostrils) and identify underlying skin diseases such as eczema, scabies and head lice.
  • Erysipelas, cellulitis and many cases of necrotising fasciitis are most frequently caused by Streptococcus pyogenes, less often by Staphylococcus aureus, enterobacteriae and anaerobes.

Skin flora

Normal skin flora includes harmless staphylococci, micrococci, diphtheroids and yeasts with aerobes on the surface and anaerobes within hair follicles. These bacteria provide some protection against pathogens by competitive interference and production of antibiotics.

Staphylococcal infections

Wound infection

Although Staphylococcus aureus is present in the nostrils of about 30% adults, it becomes invasive resulting in infection if there is an injury or other breach of the skin surface such as dermatitis, scabies or head lice.

Staphylococcal infections are contagious, requiring careful attention to hygiene.

Localised staphylococcal infections:

Surface infections tend to be itchy, deeper infections are more painful.

Management

Localised staphylococcal infections may be managed using meticulous wound care, antiseptics and topical antibiotics. Oral antibiotics may be prescribed for more extensive or recurrent infections. However, there is increasing prevalence of methicillin-resistant strains of staphylococci so antibiotics should not be prescribed for trivial reasons.

Toxins and superantigens

Some strains of Staphylococci are more pathogenic due to toxins and superantigens and may result in widespread rashes:

Scalded skin syndrome

Scalded skin syndrome


Streptococcal infections

Impetigo

Impetigo may be caused by Streptococcus pyogenes, when it is characterised by crusts and ulceration. Staphylococcus aureus is usually a co-pathogen. Streptococcal wound infections are painful with surrounding well-demarcated erythema and oedema.

Streptococcal wound infection

Erysipelas

Invasive streptococci result in erysipelas (superficial lymphatic invasion) and cellulitis (involvement of subcutis). Early infection is characterised by fever, rigors and malaise. The affected skin becomes red, swollen and painful. These infections must be treated aggressively as they are potentially fatal. Group A streptococci are uniformly sensitive to penicillin and improvement is usually seen within hours of treatment. Erysipelas and cellulitis may recur in the same site and prolonged courses of penicillin are sometimes required prophylactically.

Erysipelas

Cellulitis

Necrotising fasciitis

Necrotising fasciitis is a surgical emergency, most often due to Streptococcus pyogenes and less frequently be due to Staphylococcus aureus, enterobacteriae and other organisms. Necrotic tissue must be debrided thoroughly and immediately as a life-saving procedure.

Necrotising fasciitis

Toxins and superantigens

Some strains of Streptococci are more pathogenic due to toxins and superantigens and may result in widespread rashes:

Scarlet fever

Post-infection complications

Cutaneous Group A streptococcal infections (Streptococcus pyogenes) can result in post-streptococcal glomerulonephritis and should be treated with systemic antibiotics such as penicillin. These may not prevent later streptococcal complications due to immunological reactions to superantigens, including the skin diseases hypersensitivity vasculitis and guttate psoriasis.

Vasculitis arising 2 weeks after tonsillitis

Guttate psoriasis, arising 2 weeks after tonsillitis


Less common bacterial infections

Less common bacterial infections resulting in cutaneous signs include:

Pitted keratolysis

Erythrasma

Trichomycosis axillaris (black concretions)

Trichomycosis axillaris (yellow concretions)

Syphilitic chancre

Secondary syphilis

Tuberculosis (lupus vulgaris)

Atypical mycobacteria (M. marinum)

Meningococcal disease

Meningococcal disease