Most streptococci that contain the group A antigen are S .pyogenes. It is a prototypical human pathogen. It is used here to illustrate general characteristics of streptococci and specific characteristics of the species. S. pyogenes is the main human pathogen associated with local or systemic invasion and poststreptococcal immunologic disorders. S. pyogenes typically produces large (1 cm in diameter) zones of β-hemolysis around colonies greater than 0.5 mm in diameter. They are PYR-positive (hydrolysis of l-pyrrolidonyl-β-naphthylamide) and usually are susceptible to bacitracin.

 Morphology and Identification

 A. Typical Organisms

 Individual cocci are spherical or ovoid and are arranged in chains (Figure 1). The cocci divide in a plane perpendicular to the long axis of the chain. The members of the chain often have a striking diplococcal appearance, and rod-like forms are occasionally seen. The lengths of the chains vary widely and are conditioned by environmental factors. Streptococci are gram positive; however, as a culture ages and the bacteria die, they lose their gram positivity and can appear to be gram negative; for some streptococci, this can occur after overnight incubation.

 Most group A strains (see Table1) produce capsules composed of hyaluronic acid. The capsules are most notice able in very young cultures. They impede phagocytosis. The hyaluronic acid capsule likely plays a greater role in virulence than is generally appreciated and together with M protein was believed to be an important factor in the resurgence of rheumatic fever (RF) in the United States in the 1980s and 1990s. The capsule binds to hyaluronic-acid-binding protein, CD44, present on human epithelial cells. Binding induces disruption of intercellular junctions allowing microorganisms to remain extracellular as they penetrate the epithelium (see Stollerman and Dale, 2008). Capsules of other streptococci (eg, S. agalactiae and S. pneumoniae) are different. The S. pyogenes cell wall contains proteins (M, T, R antigens), carbohydrates (group specific), and peptidoglycans. Hairlike pili project through the capsule of group A streptococci. The pili consist partly of M protein and are covered with lipoteichoic acid. The latter is important in the attachment of streptococci to epithelial cells.

fig1. Streptococci grown in blood culture showing gram-positive cocci in chains. Original magnification × 1000.

table1. Characteristics of Medically Important Streptococci

B. Culture

Most streptococci grow in solid media as discoid colonies, usually 1–2 mm in diameter. S. pyogenes is β-hemolytic (Figure 2); other species have variable hemolytic characteristics (see Table1).

fig2. Group A β-hemolytic streptococci (Streptococcus pyogenes) after growth overnight on a 10-cm plate with 5% sheep blood agar. The small (0.5–1 mm diameter) white colonies are surrounded by diffuse zones of β-hemolysis 7–10 mm in diameter. (Courtesy of H Reyes.)

 C. Growth Characteristics

 Energy is obtained principally from the utilization of glucose with lactic acid as the end product. Growth of streptococci tends to be poor on solid media or in broth unless enriched with blood or tissue fluids. Nutritive requirements vary widely among different species. The human pathogens are most exacting, requiring a variety of growth factors. Growth and hemolysis are aided by incubation in 10% CO2 . Most pathogenic hemolytic streptococci grow best at 37°C. Most streptococci are facultative anaerobes and grow under aerobic and anaerobic conditions.

 D. Variation

Variants of the same Streptococcus strain may show different colony forms. This is particularly marked among S. pyogenes strains, giving rise to either matte or glossy colonies. Matte colonies consist of organisms that produce much M protein and generally are virulent. The S. pyogenes in glossy colonies tend to produce little M protein and are often not virulent.