Streptococcus and Enterococcus

■ Streptococci are Gram-positive, nonmotile, catalase-negative, facultatively anaerobic cocci that occur in chains or pairs. They are classified based on their hemolytic capacity (a-, b-, y-hemolysis) and the antigenicity of a carbohydrate occurring in their cell walls (Lancefield antigen).

b-hemolytic group A streptococci (S. pyogenes) cause infections of the upper respiratory tract and invasive infections of the skin and subcutaneous connective tissue. Depending on the status of the immune defenses and the genetic disposition, this may lead to scarlet fever and severe infections such as necrotizing fasciitis, sepsis, or septic shock. Sequelae such as acute rheumatic fever and glomerulonephritis have an autoimmune pathogenesis. The a-hemolytic pneumococci (S. pneumoniae) cause infections of the respiratory tract. Penicillins are the antibiotics of choice. Resistance to penicillins is known among pneumococci, and is increasing. Laboratory diagnosis involves pathogen detection in the appropriate material. Persons at high risk can be protected from pneumococcalinfections with an active prophylactic vaccine containing purified capsular polysaccharides. Certain oral streptococci are responsible for dental caries. Oral streptococci also cause half of all cases of endocarditis.

Although enterococci show only low levels of pathogenicity, they frequently cause nosocomial infections in immunocompromised patients (usually as elements of a mixed flora). ■

Streptococci are round to oval, Gram-positive, nonmotile, nonsporing bacteria that form winding chains (streptos [greek] = twisted) or diplococci. They do not produce catalase. Most are components of the normal flora of the mucosa. Some can cause infections in humans and animals.

Classification. The genera Streptococcus and Enterococcus comprise a large number of species. Table 4.2 lists the most important.

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Essentials of Human Physiology

Essentials of Human Physiology

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