21.2 Bacterial infections of the skin and eyes  (Page 5/17)

The genus Streptococcus includes important pathogens that are categorized in serological Lancefield groups based on the distinguishing characteristics of their surface carbohydrates. The most clinically important streptococcal species in humans is S. pyogenes , also known as group A streptococcus (GAS) . S. pyogenes produces a variety of extracellular enzymes, including streptolysins O and S, hyaluronidase , and streptokinase . These enzymes can aid in transmission and contribute to the inflammatory response. Starr, C.R. and Engelberg N.C. “Role of Hyaluronidase in Subcutaneous Spread and Growth of Group A Streptococcus.” Infection and Immunity 2006(7:1): 40–48. doi: 10.1128/IAI.74.1.40-48.2006. S. pyogenes also produces a capsule and M protein , a streptococcal cell wall protein. These virulence factors help the bacteria to avoid phagocytosis while provoking a substantial immune response that contributes to symptoms associated with streptococcal infections.

S. pyogenes causes a wide variety of diseases not only in the skin, but in other organ systems as well. Examples of diseases elsewhere in the body include pharyngitis and scarlet fever , which will be covered in later chapters.

Cellulitis, erysipelas, and erythema nosodum

Common streptococcal conditions of the skin include cellulitis, erysipelas, and erythema nodosum. An infection that develops in the dermis or hypodermis can cause cellulitis , which presents as a reddened area of the skin that is warm to the touch and painful. The causative agent is often S. pyogenes , which may breach the epidermis through a cut or abrasion, although cellulitis may also be caused by staphylococci. S. pyogenes can also cause erysipelas , a condition that presents as a large, intensely inflamed patch of skin involving the dermis (often on the legs or face). These infections can be suppurative , which results in a bullous form of erysipelas. Streptococcal and other pathogens may also cause a condition called erythema nodosum , characterized by inflammation in the subcutaneous fat cells of the hypodermis. It sometimes results from a streptococcal infection, though other pathogens can also cause the condition. It is not suppurative, but leads to red nodules on the skin, most frequently on the shins ( [link] ).

In general, streptococcal infections are best treated through identification of the specific pathogen followed by treatment based upon that particular pathogen’s susceptibility to different antibiotics. Many immunological tests, including agglutination reactions and ELISA s, can be used to detect streptococci. Penicillin is commonly prescribed for treatment of cellulitis and erysipelas because resistance is not widespread in streptococci at this time. In most patients, erythema nodosum is self-limiting and is not treated with antimicrobial drugs. Recommended treatments may include nonsteroidal anti-inflammatory drugs (NSAIDs), cool wet compresses, elevation, and bed rest.