The gram-positive cocci, Staphylococcus aureus and Streptococcus pyogenes (also known as group A b-hemolytic streptococcus) are not generally considered normal skin inhabitants but are temporary invaders and account for the vast majority of cutaneous bacterial infections. Methicillin-resistant Staphylococcus aureus (MRSA), a type of S. aureus that is resistant to the penicillins, is now increasingly recognized as a cause of skin infections and most often presents as recurrent furunculosis.

S. aureus can be part of the normal skin flora in some people, typically inhabiting the anterior nares, hands, and perineum, and this carriage imparts an increased risk of developing impetigo or other staphylococcal infections such as those that result from surgical or traumatic wounds. Eczematous dermatitis, a disorder in which the barrier of the skin is defective, often allows for secondary impetiginization with S. aureus (see Chapter 13: Eczema and Related Disorders).

Folliculitis refers to inflammation of the hair follicle, particularly its upper portion, and is characterized by perifollicular papules and/or pustules with or without obvious emerging hairs. Folliculitis can be the result of an infection (with bacteria, or less often viruses and fungi), inflammatory skin conditions (e.g., follicular eczema), drugs or physical irritation. Furuncles (“boils”) and carbuncles represent a deeper, more extensive progression of a bacterial folliculitis.

Hidradenitis suppurativa is not a primary bacterial infection, but it is included in this chapter because of the presence of furuncle-like lesions and the frequent occurrence of secondary infection with S. aureus.