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Skin peeling off a hand.
A newborn with staphylococcal scalded skin syndrome (SSSS), which results in large regions of peeling, dead skin. (credit: modification of work by D Jeyakumari, R Gopal, M Eswaran, and C MaheshKumar)

Impetigo

The skin infection impetigo causes the formation of vesicles, pustules , and possibly bullae , often around the nose and mouth. Bullae are large, fluid-filled blisters that measure at least 5 mm in diameter. Impetigo can be diagnosed as either nonbullous or bullous. In nonbullous impetigo, vesicles and pustules rupture and become encrusted sores. Typically the crust is yellowish, often with exudate draining from the base of the lesion. In bullous impetigo, the bullae fill and rupture, resulting in larger, draining, encrusted lesions ( [link] ).

Especially common in children, impetigo is particularly concerning because it is highly contagious. Impetigo can be caused by S. aureus alone, by Streptococcus pyogenes alone, or by coinfection of S. aureus and S. pyogenes . Impetigo is often diagnosed through observation of its characteristic appearance, although culture and susceptibility testing may also be used.

Topical or oral antibiotic treatment is typically effective in treating most cases of impetigo. However, cases caused by S. pyogenes can lead to serious sequelae (pathological conditions resulting from infection, disease, injury, therapy, or other trauma) such as acute glomerulonephritis ( AGN ), which is severe inflammation in the kidneys.

Red bumps on the upper region of the mouth.
Impetigo is characterized by vesicles, pustules, or bullae that rupture, producing encrusted sores. (credit: modification of work by FDA)

Nosocomial S. epidermidis Infections

Though not as virulent as S. aureus , the staphylococcus S. epidermidis can cause serious opportunistic infections. Such infections usually occur only in hospital settings. S. epidermidis is usually a harmless resident of the normal skin microbiota. However, health-care workers can inadvertently transfer S. epidermidis to medical devices that are inserted into the body, such as catheters, prostheses, and indwelling medical devices. Once it has bypassed the skin barrier, S. epidermidis can cause infections inside the body that can be difficult to treat. Like S. aureus , S. epidermidis is resistant to many antibiotics, and localized infections can become systemic if not treated quickly. To reduce the risk of nosocomial (hospital-acquired) S. epidermidis , health-care workers must follow strict procedures for handling and sterilizing medical devices before and during surgical procedures.

  • Why are Staphylococcus aureus infections often purulent?

Streptococcal infections of the skin

Streptococcus are gram-positive cocci with a microscopic morphology that resembles chains of bacteria. Colonies are typically small (1–2 mm in diameter), translucent, entire edge, with a slightly raised elevation that can be either nonhemolytic, alpha-hemolytic, or beta-hemolytic when grown on blood agar ( [link] ). Additionally, they are facultative anaerobes that are catalase-negative.

A micrograph of streptococcus pyogenes is shown.
Streptococcus pyogenes forms chains of cocci. (credit: modification of work by Centers for Disease Control and Prevention)

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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