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Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning easily bent or twisted, like a chain (twisted chain). Contrast this with staphylococci, which divide along multiple axes and generate grape-like clusters of cell. Streptococci are oxidase- and catalase-negative, and many are facultative anaerobes.

In 1984, many organisms formerly considered Streptococcus were separated out into Enterococci and Lactococci.

Pathogenesis

In addition to streptococcal pharyngitis (strep throat AmE), certain Streptococcus species are responsible for many cases of meningitis, bacterial pneumonia, endocarditis, erysipelas and necrotizing fasciitis (the 'flesh-eating' bacterial infections). However, many streptococcal species are non-pathogenic. Indeed, Streptococci are a necessary ingredient in Emmentaler ("Swiss") cheese. Streptococci are also part of the normal commensal flora of the mouth, skin, intestine, and upper respiratory tract of humans.

As a rule, individual species of Streptococcus are classified based on their hemolytic properties. Alpha hemolysis is caused by a reduction of iron in hemoglobin, giving it a greenish color on blood agar. Beta-only hemolysis is complete rupture of red blood cells, giving distinct, wide, clear areas around bacterial colonies on blood agar. Other streptococci are labeled as gamma hemolytic, actually a misnomer, as no hemolysis takes place.

Beta-hemolytic streptococci are further characterised via the Lancefield serotyping – based on specific carbohydrates in the bacterial cell wall. These are named Lancefield groups A to v(except I and J), although some species, such as S. pneumoniae, do not express Lancefield antigens, but these are alpha-hemolytic not the beta-hemolytic to which this scheme refers. In the medical setting, the most important groups are the alpha-hemolytic streptococci, S. pneumoniae and Streptococcus Viridans-group, and the beta-hemolytic streptococci of Lancefield groups A and B (also known as “Group A Strep” and “Group B Strep”).



Alpha-hemolytic streptococci

Pneumococci



The Viridans group: alpha hemolytic and no Lancefield antigens



Beta-hemolytic streptococci

Group A

S. pyogenes, also known as Group A Streptococcus (GAS), is the causative agent in Group A streptococcal infections, including streptococcal pharyngitis ("strep throat" AmE), acute rheumatic fever, scarlet fever, acute glomerulonephritis and necrotizing fasciitis. If strep throat is not treated, it can develop into rheumatic fever, a disease that affects the joints and heart valves. 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 A Streptococcus infection is generally diagnosed with a Rapid Strep Test (AmE) or by culture.

Group B

S. agalactiae, or GBS, causes pneumonia and meningitis in neonates and the elderly, with occasional systemic bacteremia. They can also colonize the intestines and the female reproductive tract, increasing the risk for premature rupture of membranes and transmission to the infant. The American College of Obstetricians and Gynecologists, American Academy of Pediatrics and the Centers for Disease Control recommend all pregnant women between 35 and 37 weeks gestation should be tested for GBS. Women who test positive should be given prophylactic antibiotics during labor, which will usually prevent transmission to the infant. In the UKmarker, clinicians have been slow to implement the same standards as the USmarker, Australia and Canadamarker. In the UK, only 1% of maternity units test for the presence of Group B Streptococcus. Although The Royal College of Obstetricians and Gynaecologists issued risk-based guidelines in 2003 (due for review 2006), the implementation of these guidelines has been patchy. Some groups feel that as a result over 75 infants in the UK die each year of GBS related disease and another 600 or so suffer serious infection, most of which could be prevented however this is yet to be substantiated by randomized controlled trial in the UK setting and, given the evidence for the efficacy of testing and treating from other countries, it may be that the large-scale trial necessary would receive neither funding nor ethics approval.

Group C

Includes S. equi, which causes strangles in horses, and S. zooepidemicus - a subspecies of S. equi - which causes infections in several species of mammals including cattle and horses. This can also cause death in chickens and moose.

Group D (enterococci) *variable in hemolysis

Many former Group D Streptococci have been reclassified and placed in the genus Enterococcus (including S. faecalis, S. faecium, S. durans, and S. avium). For example, Streptococcus faecalis is now Enterococcus faecalis.

The remaining non-enterococcal Group D strains include Streptococcus bovis and Streptococcus equinus.

Group G Streptococci

These streptococci are usually but not exclusively beta hemolytic. Streptococcus canis is an example of a GGS which is typically found on animals but can cause infection in humans.

Non-hemolytic streptococci

Non-hemolytic streptococci rarely cause illness. However, weakly hemolytic group D beta-hemolytic streptococci and Listeria monocytogenes should not be confused with non-hemolyticstreptococci.

Treatment



See also



References

External links




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