Streptococci are classified into the alpha, beta, or gamma groups, according to their action on blood cells. Streptococci of the alpha group (e.g., the viridans and S. pneumoniae) cause some destruction (hemolysis) of red blood cells. The beta group are more destructive of red blood cells; they also produce toxic substances that affect white blood cells and the clotting properties of blood. Members of these two groups are sometimes called hemolytic (red blood cell-destroying) streptococci. The beta-hemolytic streptococci are often further classified into lettered groups, called Lancefield groups for R. C. Lancefield, the scientist who originated the scheme in the 1930s. Group A hemolytic streptococci are responsible for most human streptococcal disease; group B hemolytic streptococci can cause serious problems in newborns. The gamma group, or nonhemolytic group, does not affect red blood cells. Enterococci (usually harmless bacteria that inhabit the intestines) and lactococci (bacteria used in starter cultures in the production of fermented dairy products) used to be considered a part of the Streptococcus genus but are now placed in their own genera.
The viridans are normal inhabitants of the body and are usually harmless; however, they can contribute to tooth decay. Streptococcus pneumoniae is the most common cause of otitis media in children. It can also cause meningitis and pneumonia. The S. pneumoniae diseases are sometimes referred to as pneumococcal diseases. The development of drug-resistant strains of pneumococci has caused concern in the medical community. Vaccination against pneumococcal pneumonia is recommended for very young children and older persons; the vaccine inoculates against the most prevalent strains of S. pneumoniae.
Group A hemolytic streptococci cause over a dozen diseases, including some pneumonias, erysipelas (a generalized body infection), upper respiratory infections, wound infections, and puerperal fever. Scarlet fever is also a streptococcal, or strep, infection; the rash is a response to a toxin produced by the bacteria that cause strep throat. Rheumatic fever follows an initial Group A streptococcal infection: proteins of the streptococcal cells stimulate antibody formation by the body (see immunity), and these antistreptococcal antibodies are believed to react with and damage many tissues of the body, especially heart muscle. Kidney disease (acute glomerulonephritis) is another complication of streptococcal infections. Some extremely serious Group A streptococcal infections began to emerge or reemerge in the late 1980s. Toxic streptococcal syndrome was first described in 1987. It is a rapidly progressing infection, similar to septicemia, that usually infects people in their 20s or 30s. Necrotizing fasciitis is a quickly spreading infection of the flesh and muscle caused by toxins released by a strain of Group A streptococcal bacteria that have been invaded by bacteriophages. Such bacteria are popularly called "flesh-eating bacteria."
Group B streptococci are a common cause of infection in babies, pregnant women, the elderly, and immunologically compromised adults. They are especially serious in newborns, in whom they can cause sepsis, meningitis, or pneumonia. Group B streptococci are often present in people who show no symptoms of disease; these people are said to be "colonized." Many infants are colonized before or during birth by mothers who unknowingly carry the bacteria. A small percentage of these develop disease, which can be life-threatening or can lead to lifelong neurological problems.
See M. P. Starr et al., ed., The Prokaryotes: A Handbook on Habitats, Isolation and Identification of Bacteria (1981).
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning easily bent or twisted, like a chain. Contrast this with staphylococci, which divide along multiple axes and generate grape-like clusters of cells.
As a rule, individual species of Streptococcus are classified based on their hemolytic properties (breakdown of red blood cells in a laboratory). Alpha hemolysis is caused by a reduction of iron in hemoglobin, giving it a greenish color on blood agar. Beta-only hemolysis is complete rupture of red blood cells, giving distinct, wide, clear areas around bacterial colonies on blood agar. Other streptococci are labeled as gamma hemolytic, actually a misnomer, as no hemolysis takes place.
Beta-hemolytic streptococci are further characterised via the Lancefield serotyping – based on specific carbohydrates in the bacterial cell wall. These are named Lancefield groups A to T, although some species, such as S. pneumoniae, do not express Lancefield antigens. See the related article on Rebecca Lancefield. In the medical setting, the most important groups are the alpha-hemolytic streptococci, S. pneumoniae and Streptococcus Viridans-group, and the beta-hemolytic streptococci of Lancefield groups A and B (also known as “Group A Strep” and “Group B Strep”).
Group A Strep infection is generally diagnosed with a Rapid Strep Test or by culture.
Non-hemolytic streptococci rarely cause disease. However, weakly hemolytic group D beta-hemolytic streptococci and Listeria monocytogenes should not be confused with non-hemolytic streptococci.