Bacterial Skin Diseases

Common skin bacteria Staphylococcus and Streptococcus can become pathogenic under certain conditions. For example, cellulitis is a bacterial infection of subcutaneous connective tissue that is triggered when an open wound is invaded by bacteria, typically streptococci or staphylococci. Folliculitis is inflammation and infection of hair follicles typically caused by a bacterium. Staphylococcus aureus is the primary cause of folliculitis that begins after damage from shaving or other irritation.

Necrotizing fasciitis is a skin infection that can be caused by Group A Streptococcus and Staphylococcus aureus, in addition to various other organisms, including Bacteroides, Klebsiella, Clostridium, Pseudomonas, and E. coli. Necrotizing fasciitis has been called "flesh-eating disease" by the media because of the rapid tissue death (gangrene) it causes when the infectious agent gains entrance to the soft tissues through a break in the skin, intravenous drug use, or a surgical complication. Mortality is high due to the speed with which it spreads in the body, causing septic shock and ultimately organ failure even with antibiotic treatment. Early signs of infection include tissue redness, swelling, and tenderness, and a clue to diagnosing this disease is in recognizing the patient complaint of pain out of proportion to the local skin findings. The keys to treatment include early recognition of the disease, broad-spectrum antibiotics, and surgical debridement, or removal of infected/dead tissue, from the affected areas.

When skin pores become blocked, the usually harmless commensal bacterium Propionibacterium acnes becomes problematic. This occurs in the skin condition acne vulgaris, or simply, acne. Although acne is seen in people of all ages, it is most common in the teenage years due to hormonal changes that affect hair follicle maturation. As a result, excess sebum produced from the oil glands of the skin mixes with dead skin cells, forming a plug. P. acnes will continue to multiply within the blocked oil gland, and the skin swells and becomes inflamed, red, and tender. Acne can be self-limited or can be treated with cleansers containing benzoyl peroxide and antibiotics such as doxycycline. Other treatments are directed at increasing the turnover of skin cells, meaning sloughed-off skin cells spend less time within the skin glands and cannot contribute to blocking the gland. These medicines are called retinoids and include tretinoin and adapalene.

Pseudomonas aeruginosa and Staphylococcus aureus are the two most common causes of otitis externa, also known as swimmer's ear, although other organisms including fungi have also been implicated. Otitis externa is the infection and inflammation of the outer ear canal. Typically, the ear canal becomes very itchy, swollen, and tender. Pain is the most common symptom associated with otitis externa. Conditions that predispose an individual to developing this condition include but are not limited to cerumen, or wax, buildup; psoriasis; and moisture in the ear from humidity or swimming/showering. Treatment consists of a topical antibiotic from the aminoglycoside or fluoroquinolone family, in the form of a drop placed in the ear. Corticosteroids may also be given if the ear is severely inflamed, and treatment is usually given for seven days.

Two of the most common bacterial skin microbes are Staphylococcus aureus and Streptococcus pyogenes. Both S. aureus and S. pyogenes are capable of causing severe infections under the right conditions and can cause impetigo. Impetigo is a highly contagious infection of the epidermis, the outer layers of the skin, that is common in young children. Impetigo spreads via direct or indirect contact. It can resolve on its own, although antibiotics are often given to expedite resolution.

Primary impetigo occurs when the bacteria invade normal healthy skin without a site of entry (e.g., a cut or wound). Secondary impetigo occurs when the skin barrier is interrupted by a wound or other infection and the bacteria can then enter the skin.

Impetigo infections can be nonbullous, bullous, or ulcerative. Seventy percent of impetigo infections are nonbullous and are characterized by small, red blisters appearing all over the body, especially around the mouth, nose, or other extremities. These blisters burst and ooze pus, and then crust over. Thirty percent of impetigo cases are bullous. A bullous skin disorder is characterized by large fluid-filled vesicles called bullae that form on the skin and eventually burst, leaving behind a yellow crust. Bullous impetigo infections are caused by a strain of Staphylococcus that produces toxin A. Toxin A breaks the skin down so the bacteria can gain entry. This type of skin disorder is very localized, only affecting a particular area.