Streptococcus is a
genus of
spherical Gram-positive bacteria belonging to the
phylum Firmicutes[2] 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
cells. Most streptococci are
oxidase- and
catalase-negative, and many are
facultative anaerobes.
In 1984, many organisms formerly considered
Streptococcus were separated out into the
genera Enterococcus and
Lactococcus.
[3]
Pathogenesis
Streptococcal classification.
In addition to
streptococcal pharyngitis (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 nonpathogenic. Indeed, streptococci are a necessary
ingredient in
Emmentaler ("Swiss") cheese. Streptococci are also part of the normal
commensal microbiome 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.
[4]
Alpha hemolysis is caused by an oxidation 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. Other streptococci are labeled as
gamma-hemolytic, actually a
misnomer, as no hemolysis takes place.
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”).
Beta-hemolytic streptococci are further characterised via the
Lancefield serotyping – based on specific carbohydrates in the bacterial cell wall.
[5] These are named Lancefield groups A to V (except I and J).
Alpha-hemolytic
Pneumococci
Inflammation is thought to be the major cause of how pneumococcus
causes disease, hence the inflammatory nature of the diagnoses
associated with it [ above ].
The Viridans group: alpha-hemolytic
Beta-hemolytic
[edit] 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.
Strep. pyogenes is the other major cause of streptococcal infection in
humans, after pneumococcus. However, rather than being directly invasive
and inflammatory, it seems to cause local infection, but then its other
actions are via toxins: whether affecting kidneys in post strep
glomerulonephritis, heart valves in rheumatic fever, the scarlet skin of
scarlet fever or dissolving tissue in necrotizing fasciitis. 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 or by culture.
Rheumatic fever, a disease that affects the
joints,
kidneys and
heart valves,
is a consequence of untreated strep A infection caused not by the
bacterium itself. Rheumatic fever is caused by the antibodies created by
the immune system to fight off the infection cross-reacting with other
proteins in the body. This cross-reaction causes the body to essentially
attack itself and leads to the damage above.
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 that 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.
[6] 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
Streptococcus.
[7]
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. 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;
[8]
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.
[9]
Group C
This group includes
S. equi, which causes
strangles in horses,
[10] and
S. zooepidemicus -
S. equi is a clonal descendent or biovar of the ancestral
S. zooepidemicus - which causes infections in several species of mammals, including cattle and horses.
Streptococcus dysgalactiae
is also a member of Group C, β-haemolytic streptococci that can cause
pharyngitis and other pyogenic infections similar to Group A
streptococci.
Group D (enterococci)
Many former Group D streptococci have been reclassified and placed in the genus
Enterococcus (including
Enterococcus faecalis,
Enterococcus faecium,
Enterococcus durans, and
Enterococcus avium).
[11] For example,
Streptococcus faecalis is now
Enterococcus faecalis.
The remaining nonenterococcal Group D strains include
Streptococcus bovis and
Streptococcus equinus.
Nonhemolytic streptococci rarely cause illness. However, weakly hemolytic group D beta-hemolytic streptococci and
Listeria monocytogenes (which is actually a Gram-positive bacillus) should not be confused with nonhemolytic streptococci.
Group F streptococci
Group F streptococci were first described in 1934 by Long and Bliss amongst the "minute haemolytic streptococci".
[12] They are also known as
Streptococcus anginosus (according to the Lancefield classification system) or as members of the
S. milleri group (according to the European system).
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.
