Streptococci

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Streptococci are facultatively anaerobic, Gram-positive organisms that often occur as chains or pairs and are catalase-negative (staphylococci are catalase positive). Streptococci are subdivided into groups by antibodies that recognize surface antigens.These groups may include one or more species. The most important groupable streptococci are A, B and D. Among the groupable streptococci, infectious disease (particularly pharyngitis) is caused by group A which is thus emphasized here. Streptococcus pneumoniae (a major cause of human pneumonia) and Streptococcus mutans and other so-called viridans streptococci (among the causes of dental caries) do not possess group antigens.

Three types of hemolysis reaction are seen after growth of streptococci on sheep blood agar (alpha, beta, gamma). alpha refers to partial hemolysis with a green coloration (from production of an unidentified product of hemoglobin) seen around the colonies; beta refers to complete clearing and gamma means there is no lysis. Group A and group B streptococci are beta hemolytic, whilst D are usually alpha or gamma. Streptococcus pneumoniae and viridans ("green") streptococci are α hemolytic. Thus the hemolysis reaction is important in grouping streptococci. The hemolysis reaction along with one physiologic characteristic is sufficient for a presumptive clinical identification.

Group A streptococcus (S. pyogenes)

This organism traditionally causes suppurative, but non-invasive pharyngitis, and less frequently the skin infection, impetigo. In the middle part of the 1900's, the serious complications of group A streptococcal infections began to decline dramatically and had greatly decreased by the 70's. Thus, interest in this organism waned. In the 80's and 90's, there has been an upsurge in classical "rheumatic fever" (a non-suppurative disease of the heart) but also new forms of streptococcal disease which includes both "invasive" bacteremia, a toxic shock-like syndrome (as seen with S. aureus) and so-called "flesh eating" bacteria.

Group A streptococcal infections affect all ages with peak incidence at 5-15 years of age. The serious complications (including rheumatic fever and invasive bacteremia) were felt to affect primarily those with some underlying defect in their immune system (including infants, elderly people and those immunocompromised). However, it is clear now that previously healthy children and adults are definitely at risk of serious complications.

Rheumatic fever. Rheumatic fever, is an inflammatory disease affecting primarily the heart and joints. Although severe it can take an extended period of time to develop. The mechanism of chronic immunopathology of rheumatic fever is not resolved. M protein cross-reacts with heart myosin leading to autoimmunity. Also the group A streptococcal cell wall is highly resistant to degradation in the host. These antigens persist for months in vivo and experimentally elicit diseases that resemble rheumatic arthritis and carditis. Rheumatic arthritis should not be confused with the most common rheumatic disease - rheumatoid arthritis. Early termination of throat infections with penicillin therapy decreases the incidence of the subsequent development of rheumatic carditis.

Acute glomerulonephritis. This is an immune complex disease of the kidney.

Scarlet fever. The characteristic rash is caused by erythrogenic (pyrogenic) toxins which are phage encoded.

Bacteremia and toxic-shock. The newly described invasive (and sometimes fatal) forms of the disease with a toxic shock-like disease (including rash, fever and shifting of fluid from the bloodstream to peripheral tissues with resulting edema) and/or necrotizing myositis and fasciitis. Production of pyrogenic toxins (A, B and C) are a hallmark of these strains. Pyrogenic toxin is a superantigen (a mitogen) for T cells causing non-specific activation of the immune system. This may be involved in the pathogenesis. This disease is still uncommon but can progress very quickly (a few days) and is life-threatening.

General features in pathogenesis

The identity of the adhesin allowing adhesion to the respiratory epithelium (via fibronectin) is somewhat controversial. Lipoteichoic acid is localized in the cell membrane of many bacteria. For group A streptococci, much is also present in the fimbriae on the cell exterior. Classical work suggests lipoteichoic acid is the group A streptococcal adhesin although more recently a role for an "F (fibronectin-binding) protein" has been suggested.

Group A streptococci in the absence of fibrinogen fix complement to the peptidoglycan layer and, in the absence of antibodies, are not phagocytosed. The M protein (also found in fimbriae) binds fibrinogen from serum and blocks the binding of complement to the underlying peptidoglycan. This allows survival of the organism by inhibiting phagocytosis. However, in immune individuals, neutralizing antibodies reactive with M protein elicit phagocytosis which results in killing of the organism. This is the major mechanism by which immunity is able to terminate group A streptococcal infections. M protein vaccines are thus a major candidate for use against rheumatic fever. The capsule of group A streptococci classically was stated to have limited anti-phagocytic activity. Many of the newly described virulent strains are highly mucoid and the capsules are important in pathogenesis.

Unfortunately, certain M protein types cross-react antigenically with the heart and may be responsible for rheumatic carditis. The fear of autoimmunity has rightly inhibited the use of group A streptococcal vaccines. However, distinct protective versus cross-reactive epitopes have been defined and the availability of a vaccine appears likely. M proteins vary antigenically between strains; thus immunity to one M protein does not imply general immunity to all S. pyogenes strains. M typing along with other antigens (T and R) are used for serotyping.

Laboratory diagnosis

1. Direct detection - the antigen is extracted from a throat swab. The antigen extract will then bind with antibody specific to the group A streptococcal carbohydrate. This has classically involved agglutination of antibody coated beads. However, simpler tests have been recently introduced. Results are available within minutes.

2. Lancefield grouping of isolated beta hemolytic colonies (see above).

3. Colonies are beta hemolytic and their growth is inhibited by bacitracin (presumptive diagnosis).

4. Patient serum shows antibodies to streptolysin O or other streptococcal antigens. This is important if delayed clinical sequelae occur.

Beta hemolysis is caused by two hemolysins O and S; the former is inactive in the presence of oxygen. Thus, stabbing of the plate increases the intensity of the hemolysis reaction.

Group B streptococcus (S. agalactiae)

These organisms cause neonatal meningitis and septicemia after transmission from the normal vaginal flora of the mother.

The organism can be identified on the basis of beta hemolysis, hydrolysis of hippurate and the CAMP reaction. CAMP is an abbreviation for the names of the four individuals who originally described the test. Group B streptococci produce a factor that increases beta hemolysis of an S. aureus indicator strain.

Group D streptococcus

Growth on bile-esculin produces a black precipitate derived from esculin: many other bacteria will not grow in the presence of bile. Group D streptococci are divided into those that will grow in 6.5% saline (enterococci) and those that will not (non-enterococci). Enterococci much more commonly cause human disease than non-enterococci. Enterococci are often resistant to penicillin. Enterococci are distantly related to other streptococci and have been moved into the genus Enterococcus; the most commonly isolated is E. (S.) faecalis. As the name implies enterococci are found in the gut flora and infection often follows from fecal contamination. A significant cause of urinary tract infections (much less common than E. coli) and also opportunistic infections (including intra-abdominal, septicemia and endocarditis). Colonies are usually alpha or gamma hemolytic.

Other beta hemolytic groups

Groups C and G (and rarely group F) occasionally cause human disease (particularly pharyngitis).

Minute colony streptococci

The normal human flora contains organisms that may be group A, C, F or G or are non-groupable (S. anginosus/S. milleri). Their role in human disease is unclear.

Viridans streptococci

These are a diverse group of species commonly found orally (including S. mutans) and cause endocarditis after release into the bloodstream from tooth extraction. They are also involved in dental caries. They are alpha hemolytic and negative for other tests described above. They are non-groupable.