Skin Diseases

Bacterial Skin Diseases

Skin Infections

Bacterial skin infections are often opportunistic, caused by native skin species, and mild, though there are serious bacterial skin diseases requiring rapid and aggressive treatment.

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.

Various skin conditions are caused by microbes that are not initially present on the skin naturally and must be transmitted. Transmission is the passing of a disease-causing pathogen from one infected person or group to another by either direct person-to-person contact or indirect contact, in which the pathogen is acquired from an inanimate object. Whenever a surface is touched by human skin, some of the resident biota are left behind, and since some of these organisms are adapted to living on the dry, saline portions of the skin, they can survive for long periods of time. Because of this, pathogens that cause skin infections are easily transmitted indirectly.

Staphylococcus aureus versus Streptococcus pyogenes

Staphylococcus aureus and Streptococcus pyogenes cause similar skin infections and should be differentiated before treatment.

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.

Skin Lesions

Lesion Type Description Appearance
Pustule Small pimple or blister on the skin containing pus; usually from fungal infections
Ulcer Crater-like opening sore on the skin caused by poor blood flow; caused by infection, immobility, or blood flow diseases, like diabetes
Crust Dried oils, pus, or blood on the skin; caused by drying of blood after scabs have been removed
Cyst Hollow vesicle containing some form of liquid secretion (e.g. pus); caused by buildup of liquid in a particular area
Macule Flat, discolored area on surface of skin; caused by prolonged ultraviolet light exposure; called age spots
Papule Small, raised pimple or swelling that may contain pus; caused when hair follicles are blocked with dirt and oil
Vesicle Small fluid-filled sacs called blisters; caused when fluid gets trapped under the epidermis
Wheal Elevated, discolored patch of skin that becomes itchy; caused by insect bites, adverse reactions to contact, or skin diseases like eczema

Several types of skin surface abnormalities occur with regularity in response to various infections.

These two gram-positive bacteria can be distinguished in the lab under the microscope, with selective media, and through biochemical tests. Streptococcus characteristically grows in chains, while Staphylococcus grows in clusters. These growth patterns are directly observable under a microscope after Gram (or other) staining is performed. A useful biochemical test takes advantage of Staphylococcus's ability to produce an enzyme called catalase that allows them to decompose hydrogen peroxide to water and oxygen. This enzyme is produced to protect their DNA from damage by reactive oxygen species. The catalase test involves placing a small amount of bacterial culture on a glass microscope slide and covering it with hydrogen peroxide. Staphylococci will decompose the hydrogen peroxide into oxygen, which will produce bubbles, but streptococci will not. One type of selective media used to distinguish S. aureus from S. pyogenes is mannitol salt agar (MSA). When plated on MSA, which has a red-pink color, S. aureus will grow and change the color of the medium to yellow. Streptococcus pyogenes does not grow well on MSA and does not cause a color change.

MRSA Infection

MRSA (methicillin-resistant Staphylococcus aureus) is found naturally in 2 out of every 100 people in the United States and is responsible for many cases of life-threatening hospital-acquired infections.
A life-threatening skin infection is caused by MRSA (methicillin-resistant Staphylococcus aureus). MRSA is found naturally in 2% of people in the United States and is responsible for many cases of hospital-acquired infections. MRSA strains of Staphylococcus aureus differ genetically from non-MRSA strains in that they are resistant to some commonly used antibiotics, specifically beta-lactam antibiotics such as methicillin, penicillin, and cephalosporins. MRSA infections are typically nosocomial, acquired in hospitals, because bacterial strains there are more frequently exposed to antibiotics and thus are more likely to develop resistance. Initially, a MRSA skin infection resembles that of other staph infections, causing problems such as folliculitis, cellulitis, and impetigo. It also formed pustules, which are small pimples or blisters on the skin containing pus. Because MRSA is so difficult to treat, however, it can spread from the skin to the bloodstream, where it can reach the lungs, heart, bones, and joints. Treatment can sometimes require surgery to remove infected bone or other tissues. Advanced MRSA infection can trigger toxic shock, an extreme response to bacterial chemicals called superantigens that can result in tissue damage, organ failure, and death.
A MRSA (methicillin-resistant Staphylococcus aureus) infection often begins as a rapidly appearing skin abscess. Staphylococcus aureus (scanning electron microscope, 20,000x), including MRSA, can spread to many regions of the body, causing great harm and sometimes death.
Credit: CDC/Janice Haney Carr (bottom)