streptococcus (strĕp´təkŏk´əs), any of a group of gram-positive bacteria, genus Streptococcus, some of which cause disease. Streptococci are spherical and divide by fission, but they remain attached and so grow in beadlike chains. The incidence and severity of streptococcal diseases decreased dramatically after the introduction of antibiotics (penicillin, erythromycin, and selected cephalosporins are all effective against the organisms), but the medical community was shaken by the arrival in the late 1980s of several severe forms of streptococcal infection and by the emergence of several drug-resistant strains (see drug resistance).
Types of Streptococci
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 several 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, such as septicemia and meningitis, especially 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.
S. pneumoniae and Viridans Infections
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 Streptococcal Infections
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. Streptococcal toxic shock syndrome is a rapidly progressing infection, similar to septicemia or toxic shock syndrome, that usually infects people in their 20s or 30s. It causes blood pressure to fall rapidly and organs to fail. Necrotizing fasciitis is a quickly spreading infection of the flesh and muscle caused by toxins released by S. pyrogenes. Such bacteria are popularly called "flesh-eating bacteria."
Group B Streptococcal Infections
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).