Scarlet fever awes me, and is above my aim. I leave it to the professional and graduated homicides. —Sydney Smith, 1833
Bacteria of the genus Streptococcus are Gram-positive cocci typically arranged in chains. In addition to relatively harmless members of the oropharyngeal flora, the genus includes three of the most important pathogens of humans. The group A streptococcus (S pyogenes) is the cause of “strep throat,” which can lead to scarlet fever, rheumatic fever, and rheumatic heart disease; the ability of some strains to cause catastrophic deep tissue infections led British tabloids to apply the gory label “flesh-eating bacteria.” The group B streptococcus (S agalactiae) is the most common cause of sepsis in newborns and the pneumococcus (S pneumoniae) a leading cause of both pneumonia and meningitis in persons of all ages.
Streptococci stain readily with common dyes, demonstrating that coccal cells are generally smaller and more ovoid in shape than staphylococci. They are usually arranged in chains with oval cells touching end to end, because they divide in one plane and tend to remain attached (Figure 25–1). Length may vary from a single pair to continuous chains of over 30 cells, depending on the species and growth conditions. Medically important streptococci are not acid-fast, do not form spores, and are nonmotile. Some members form capsules composed of polysaccharide complexes or hyaluronic acid.
Group A streptococcus (GAS) Gram stain. Note the oval cocci chaining end to end (arrow). (Image contributed by Professor Shirley Lowe, University of California, San Francisco School of Medicine, with permission.)
Oval cells arranged in chains end to end
Streptococci grow best in enriched media under aerobic or anaerobic conditions (facultative). Sheep blood agar is preferred because it satisfies the growth requirements and also serves as an indicator for patterns of hemolysis. The colonies are small, ranging from pinpoint size to 2 mm in diameter, and they may be surrounded by a zone where the erythrocytes suspended in agar have been hemolyzed. When the zone is clear, this state is called β-hemolysis (Figure 25–2). When the zone is hazy with a green discoloration of the agar, it is called α-hemolysis. Streptococci are metabolically active, attacking a variety of carbohydrates, proteins, and amino acids. Glucose fermentation yields mostly lactic acid. In contrast to staphylococci, streptococci are catalase-negative.
β-Hemolysis. Colonies of group A streptococci (GAS) on sheep blood agar plates are surrounded by a zone of complete clearing of the RBCs suspended in the agar. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)
β-Hemolysis is clear