Etiology

Most streptococci that contain cell wall antigens of the Lancefield group A (Table 17-1) are known as Streptococcus pyogenes. Members of this species are almost always beta-hemolytic streptococci. S. pyogenes is the most common causative agent of pharyngitis and its resultant disorder, scarlet fever, and the skin infection, impetigo. The most common type of bacteria causing necrotizing fasciitis is S. pyogenes.

In terms of human morbidity and mortality worldwide, however, the role of S. pyogenes in the subsequent development of complications such as acute rheumatic fever and poststreptococcal glomerulonephritis is more important. Other S. pyogenes–associated infections include otitis media in children, sinusitis in adults, and osteomyelitis, septic arthritis, neonatal septicemia, and rare cases of pneumonia.

Necrotizing fasciitis is a rare infection that can destroy skin and soft tissues, including fat and the tissue-covering muscles (fascia). Because these tissues die rapidly, a person with necrotizing fasciitis is sometimes said to be infected with so-called flesh-eating bacteria. A highly invasive group A streptococcal infection is associated with toxic shock syndrome.

Morphologic Characteristics

S. pyogenes is a gram-positive coccus and the serotype most frequently associated with human infection. Lancefield divided these beta-hemolytic streptococci into serogroups A through O on the basis of the immunologic action of the cell wall carbohydrate (Fig. 17-1).

Structures called fimbriae arise near the plasma membrane and project through the cell wall and capsule. These processes contain important surface components of the streptococcus. Lipoteichoic acid on the fimbriae is important in the organism’s adherence to human epithelium and the initiation of infection. The M and R antigens, which are structurally similar but immunologically distinct, are also found on the fimbriae. R antigen has no known biological role.

M protein, a cell protein found in association with the hyaluronic capsule, is a major virulence factor of S. pyogenes. Strains of S. pyogenes that lack M protein cannot cause infection. M protein inhibits phagocytosis and antibody synthesized against M protein provides type-specific immunity to group A streptococci. In addition, M protein is the basis for a subclassification of group A streptococci into more than 60 M serotypes.

Epidemiology

S. pyogenes is one of the most common and ubiquitous of human pathogens. It is found in the human respiratory tract and is always considered a potential pathogen. Upper respiratory infections caused by S. pyogenes occur most frequently in school-age children and are uncommon in children younger than 3 years. No gender or race predilection has been described.

Infection is spread by contact with large droplets produced in the upper respiratory tract. Although not as common, foodborne and milkborne epidemics do occur. Crowding enhances the spread of microorganisms.

A number of individuals, particularly school-age children, carry S. pyogenes without signs of illness. Carriers have positive cultures without serologic evidence of infection. If a person carries the organisms in the pharynx for prolonged periods after untreated infection, the number of organisms carried and their ability to produce M protein decline during carriage. This results in a progressive decline in the likelihood of spreading infection to others.

The incidence of a major complication of S. pyogenes, rheumatic fever, has decreased in the United States. It occurs primarily in the rural South and in areas of crowding and lower socioeconomic status. The incidence of rheumatic fever is 2% to 3% in epidemics and 0.1% to 1% after sporadic cases of streptococcal infection. The probability of developing rheumatic fever is age related, with younger patients more likely to develop carditis than older persons.

Rheumatic fever and resultant valvular heart disease, however, are syndromes of major importance among children in developing nations. Patients with a history of rheumatic heart disease resulting from rheumatic fever are at a significantly increased risk of developing cardiac malfunction and endocarditis later. The risk of recurrent rheumatic fever depends on factors such as the age of the patient at previous recurrences, length of time since the last recurrence, and presence of carditis. In addition, patients who develop streptococcal glomerulonephritis are at risk of later development of renal failure.

Signs and Symptoms

S. pyogenes causes a wide variety of infections, most often acute pharyngitis (strep throat) and upper respiratory infection, as well as impetigo (pyoderma). Other manifestations of infection with S. pyogenes include sinusitis, otitis, peritonsillar and retropharyngeal abscess, pneumonia, scarlet fever, erysipelas, cellulitis, puerperal sepsis, and gangrene. A concern still exists that group A streptococcus may be acquiring greater virulence.