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. Contrast this with staphylococci, which divide along multiple axes and generate grape-like clusters of cells.
Streptococci are oxidase- and catalase-negative.
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). 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 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 streptococci
Pneumococci
Viridans and others
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"), 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 Strep infection is generally diagnosed with a Rapid Strep Test 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
UK, clinicians have been slow to implement the same standards as the
US,
Australia and
Canada. In the UK, only 1% of maternity units test for the presence of Group B Strep. Although The Royal College of Obstetricians and Gynaecologists issued risk-based guidelines in 2003 (due for review 2006), the implentation 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 RCT 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, 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. 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.
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
External links
