Streptococcus

Streptococcus is a genus of spherical Gram-positive bacteria, belonging to the phylum Firmicutes^[1] 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 streptos, meaning easily bent or twisted, like a chain. Contrast this with staphylococci, which divide along multiple axes and generate grape-like clusters of cells.

Streptococci are oxidase– and catalase–negative.

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Pathogenesis

In addition to strep throat, 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. 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 (breakdown of red blood cells in a laboratory).^[2] 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.^[3] These are named Lancefield groups A to T, although some species, such as S. pneumoniae, do not express Lancefield antigens. See the related article on Rebecca Lancefield. 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 Streptococcus

Pneumococci

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

Viridans and Others

• S. mutans, a contributor to dental caries
• S. viridans, a cause of endocarditis and dental abscesses
• S. thermophilus, used in the manufacture of some cheeses and yogurts
• S. constellatus, occasional human pathogen, notable as colonies grown on blood agar smell strongly of caramel

Beta-Hemolytic Streptococci

Group A

S. pyogenes (also known as GAS) is the causative agent in Group A streptococcal infections, (GAS) including strep throat, 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 B

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 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. Those that test positive should be given prophylactic antibiotics during labor, which can prevent transmission to the infant.^[4] In the UK, clinicians have been slow to implement the same standards as the US, Australia and Canada. Only 1% of maternity units test for the presence of Group B Strep.^[5] Some groups feel that as a result over 75 infants in the UK die each year of GBS related disease,^[6] however this is yet to be substantiated by RCT in the UK setting.^[7]

Group C

Includes S. equi, which causes strangles in horses,^[8] and S. zooepidemicus, which causes infections in several species of mammals including cattle and horses. This can also cause death in chickens and moose. Many mountaineers from Canada have found corpses of moose lying in the middle of the road; post-mortem tests have found group c streptococcus in their blood.

Group D (Enterococci) *variable in hemolysis

Many former Group D streptococci have been reclassified and placed in the genus Enterococcus (includes S. faecalis, S. faciem, S. durans, and S. avium).^[9] For example, Streptococcus faecalis is now Enterococcus faecalis.

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

Non-Hemolytic Streptococci

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

References