streptococcus
streptococcus, (genus Streptococcus), genus of gram-positive, spherical bacteria, many species of which are normal inhabitants of the human body, while others are important causes of infectious disease.
Characteristics and ecology
The name Streptococcus is derived from the Greek streptos, meaning “twisted,” and kokkos, meaning “berry,” in reference to the bacteria’s tendency to form chains resembling strings of beads. This arrangement results from repeated cell division in a single plane, with daughter cells remaining attached after division. Indeed, members of the genus typically occur in pairs or chains and are distinguished by their thick cell walls, lack of motility, and diverse metabolic and pathogenic (disease-causing) properties.
Species of Streptococcus occupy a wide range of ecological niches. Many form part of the normal microbiota of the mouth, throat, intestines, and female reproductive tract, where they usually coexist harmlessly with the host. Others are important pathogens capable of causing infections ranging from mild pharyngitis (sore throat) to life-threatening invasive disease, while a small number are used commercially in the production of fermented dairy products.
" height="20" alt="HEW Logo">
Medically important streptococci
The principal pathogenic streptococci are classified into several medically important groups, each associated with characteristic diseases and patterns of infection. S. pyogenes, often referred to as group A streptococcus bacteria, can cause rheumatic fever, impetigo, scarlet fever, puerperal fever, streptococcal toxic shock syndrome, strep throat, tonsillitis, and other upper respiratory infections. Necrotizing fasciitis, a rapidly spreading infection of the skin and underlying tissue caused by S. pyogenes, has been popularly referred to as the “flesh-eating disease.”
Streptococcus agalactiae, or group B streptococcus bacteria, can cause infections of the bladder and uterus in pregnant women. In newborn infants infection with the bacterium may result in sepsis (blood poisoning), meningitis (inflammation of the membranes covering the brain and spinal cord), or pneumonia.
Streptococcus pneumoniae, also called pneumococcus, is an important human pathogen that causes pneumonia, sinusitis, otitis media, and meningitis. S. mutans, belonging to the viridans species, inhabits the mouth and contributes to tooth decay.
Classification
Streptococci are classified using several complementary systems based on their biological and biochemical characteristics. In clinical microbiology, one of the most important is the hemolytic classification, which groups species according to their ability to lyse red blood cells on blood agar. Alpha-hemolytic streptococci produce partial hemolysis, creating a greenish discoloration around colonies; beta-hemolytic streptococci produce complete hemolysis; and gamma-hemolytic streptococci produce little or no hemolysis.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
Many beta-hemolytic streptococci are further differentiated by the Lancefield classification system, developed by American microbiologist Rebecca Lancefield in the 1930s. This system distinguishes bacteria according to the composition of carbohydrate antigens in their cell walls and assigns them to lettered groups, such as group A and group B. Not all streptococci possess Lancefield antigens, however. For example, S. pneumoniae and the viridans streptococci, a diverse group of alpha-hemolytic species commonly found in the mouth, are classified by other microbiological and genetic characteristics rather than by the Lancefield system.