Section 3: Bacterial Infections

Impetigo Impetigo is a primary superficial infection of the skin. It is more prevalent in humid climates and occurs most commonly in the summer months. Trauma to the skin, such as a small abrasion or insect bite, sometimes provides the site of entry for the infective bacteria. The lesions evolve from discrete small vesicles into pustules. The fluid content of the primary lesions dries into a thick yellowish crust (Fig. 3-1), and removal of the crust may reveal bright-red and shiny erosions (Fig. 3-2).

The most common cause of impetigo is Staphylococcus aureus. Because the “honey-crusted” lesions of impetigo may be caused by a combination of S aureus and Streptococcus pyogenes, systemic antibiotic therapy should be effective against both organisms. The use of topical mupirocin ointment appears to be an effective treatment and may replace the need for systemic therapy in some patients with localized lesions.

Bullous impetigo This form of impetigo consists of flaccid blisters that quickly rupture and evolve into superficial round or oval erosions with a varnished surface and minimal crust. Blisters are caused by the local effect of staphylococcal toxin. Figure 3-3 shows blisters and superficial erosions.

Figure 3-4 shows the collarettes of scale following rupture of the bullae. Bullous impetigo is associated with a pure culture of S aureus. Oral treatment with dicloxacillin or a cephalosporin is an effective mode of therapy. If methicillin-resistant Staphylococcus aureus (MRSA) is suspected, oral clindamycin is frequently recommended, and can be used pending results of culture.

Impetiginization This is the term for impetigo imposed upon preexisting dermatoses, most commonly insect bites and atopic dermatitis. Eruptions that are pruritic are particularly susceptible to secondary infection. The most common organisms are S pyogenes and S aureus. Figure 3-5 shows a case of impetiginized atopic dermatitis. The development of such “honey-crusted” lesions in a child with eczema suggests the need for systemic antibiotic therapy.

Ecthyma Ecthyma occurs when there is ulceration beneath the surface of a skin infection. If impetigo is infection by streptococci and/or staphylococci superficially in the epidermis, ecthyma is infection by the same organisms through the entire thickness of the epidermis (0.1 mm) to the upper reaches of the dermis (perhaps to a depth of 0.5 mm). Clinically, there is often a firm crust covering a superficial ulcer, surrounded by erythema.