Classification

In the early 1950s, Lancefield divided streptococci into groups based on carbohydrates present in the cell wall and designated the groups A through H and K through T. In addition, streptococci may be classified by their characteristics on culture on sheep blood agar. β-Hemolytic streptococci produce zones of clear hemolysis around each colony; α-hemolytic streptococci (Streptococcus viridans) produce a green discoloration characteristic of incomplete hemolysis; absence of hemolysis is characteristic of γ-streptococci.

Pharyngitis

The sole member of Lancefield group A is Streptococcus pyogenes. Group A streptococcus is ubiquitous in the environment but with rare exceptions is exclusively found in or on the human host. About 5% to 20% of the population harbor group A streptococcus in their pharynx, and some are colonized on their skin. This organism produces a variety of suppurative infections; however, streptococcal pharyngitis, the most common, is characterized by the onset of sore throat, fever, painful swallowing, and chilliness. These symptoms combined with submandibular adenopathy, pharyngeal erythema, and exudates correlate with positive throat cultures in 85% to 90% of cases. Sore throat without fever or any of the other signs and symptoms has a low predictive value for pharyngitis caused by group A streptococcus. Rapid strep tests correlate with positive cultures in 68% to 99% of cases, but results depend greatly on the individual performing the test as well as the bacterial colony count. Colony counts greater than 100 per plate correlated with positive rapid strep tests in 95% of patients, and counts less than 100 per plate correlated with positive rapid strep tests for only 68% of patients.