Streptococcus

Streptococcus is a genus of spherical, Gram-positive bacteria of the phylum Firmicutes. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, thus the name — from Greek streptos, meaning easily bent or twisted, like a chain. Constrast this with staphylococci which divide along multiple axes and generate grape-like clusters of cells.

In addition to strep throat, members of this genus are responsible for many cases of meningitis, bacterial pneumonia, endocarditis, erysipelas and even necrotizing fasciitis (so-called 'flesh-eating' bacterial infections). It should be noted, however, that many streptococcal species are harmless.

Streptococci are part of the normal commensal flora of the mouth, skin, intestine and upper respiratory tract of humans.

Classification of Streptococci

Individual species of Streptococcus are classified primarily based on their hemolytic properties (breakdown of red blood cells in a lab). Alpha hemolysis is caused by a reduction of iron in hemoglobin giving it a greenish color on blood agar. Beta hemolysis is complete rupture of red blood cells giving distinct, wide, clear areas around bacterial colonies on blood agar.

Lancefield serotyping - based on specific carbohydrates in the bacterial cell wall - is used for further characterizing the Beta hemolytic streptococcal species. }} These are named Lancefield groups A to O. Medically, Beta-hemolytic streptococci of Lancefield groups A and B (also known as “Group A Strep” and “Group B Strep”) are the most important. Additionally, some of the alpha-hemolytic streptococci (particularly S. pneumoniae and Streptococcus Viridans-group) cause common diseases in man.

Group A Strep

S. pyogenes (also known as GAS) is the causative agent in Group A streptococcal infections, including strep throat, acute rheumatic fever, scarlet fever, and acute glomerulonephritis. Other Streptococcus species may also possess the Group A antigen, but human infections by non-S. pyogenes GAS strains (some S. dysgalactiae subsp. equisimilis and S. anginosus Group strains) appear to be uncommon.

Group B Strep

S. agalactiae, or GBS, causes meningitis in neonates and the elderly, with occasional systemic bacteremia. They can also colonize the female reproductive tract, increasing the risk for premature rupture of membranes and transmission to the child. The CDC recommends women who are pregnant should be tested for Group B Strep and given antibiotics during labor in order to reduce the risk of transmission to the baby. In the uk, as the prevalance in the female genital tract is around 15%, antibiotics are not reccomended as the side effects of giving antibiotics to the sheer number of women who would test positive and not pass it to their babies outweighs the very few it would benefit.

Group C Strep

Includes S. equi which causes strangles in horses, and S. zooepidemicus which causes infections in several species of mammals including cattle, horses and humans.

Group D Strep (Enterococci)

Many former Group D streptococci have been reclassified and placed in the genus Enterococcus (includes S. faecalis, S. faciem, S. durans, and S. avium). }} For example, Streptococcus faecalis is now Enterococcus faecalis.

The remaining non-enterococcal Group D strains include S. bovis and S. suis.

Pneumococci

S. pneumoniae, a leading cause of bacterial pneumonia and meningitis.

Viridans and Other Streptococci

Includes S. viridans, a cause of endocarditis and dental abscesses.

References

Firmicutes

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