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''Streptococcus'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...
' (plural ) or spherical bacteria that belongs to the family
Streptococcaceae Streptococcaceae are a family of Gram-positive bacteria, placed within the order Lactobacillales. Representative genera include '' Lactococcus'', ''Lactovum ''Lactovum'' is a genus of bacteria within the family Streptococcaceae. The genus ...
, within the order
Lactobacillales Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped (bacilli) or spherical (cocci) bacteria that share common metabolic and physiological characteristics. These bact ...
(lactic acid bacteria), in the phylum
Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earl ...
.
Cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
in streptococci occurs along a single
axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...
, so as they grow, they tend to form pairs or chains that may appear bent or twisted. This differs from
staphylococci ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical ( cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultati ...
, which divide along multiple axes, thereby generating irregular, grape-like clusters of cells. Most streptococci are oxidase-negative and catalase-negative, and many are
facultative anaerobes A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...
(capable of growth both aerobically and anaerobically). The term was coined in 1877 by Viennese surgeon Albert Theodor Billroth (1829–1894), by combining the prefix "strepto-" (from ), together with the suffix "-coccus" (from Modern , from .) In 1984, many bacteria formerly grouped in the genus ''Streptococcus'' were separated out into the
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
''
Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are gram-positive cocci that often occur in pairs (diplococci) or short chains, and are difficult to distinguish from streptococci on physical char ...
'' and ''
Lactococcus ''Lactococcus'' is a genus of lactic acid bacteria that were formerly included in the genus ''Streptococcus'' Group N1. They are known as homofermenters meaning that they produce a single product, lactic acid in this case, as the major or only pr ...
''. Currently, over 50 species are recognised in this genus. This genus has been found to be part of the salivary microbiome.


Pathogenesis and classification

In addition to streptococcal pharyngitis (strep throat), certain ''Streptococcus'' species are responsible for many cases of pink eye,
meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
,
bacterial pneumonia Bacterial pneumonia is a type of pneumonia caused by bacterial infection. Types Gram-positive ''Streptococcus pneumoniae'' () is the most common bacterial cause of pneumonia in all age groups except newborn infants. ''Streptococcus pneumoniae'' ...
,
endocarditis Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves. Other structures that may be involved include the interventricular septum, the chordae tendineae, the mural endocardium, or the ...
,
erysipelas Erysipelas () is a relatively common bacterial infection of the superficial layer of the skin ( upper dermis), extending to the superficial lymphatic vessels within the skin, characterized by a raised, well-defined, tender, bright red rash, t ...
, and
necrotizing fasciitis Necrotizing fasciitis (NF), also known as flesh-eating disease, is a bacterial infection that results in the death of parts of the body's soft tissue. It is a severe disease of sudden onset that spreads rapidly. Symptoms usually include red or p ...
(the 'flesh-eating' bacterial infections). However, many streptococcal species are not pathogenic, and form part of the
commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
human microbiota of the mouth, skin, intestine, and upper respiratory tract. Streptococci are also a necessary ingredient in producing Emmentaler ("Swiss") cheese. Species of ''Streptococcus'' are classified based on their
hemolytic Hemolysis or haemolysis (), also known by several other names, is the rupturing (lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo o ...
properties. Alpha-hemolytic species cause oxidization of iron in
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
molecules within red blood cells, giving it a greenish color on blood agar.
Beta-hemolytic ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs ...
species cause complete rupture of red blood cells. On blood agar, this appears as wide areas clear of blood cells surrounding bacterial colonies. Gamma-hemolytic species cause no hemolysis. Beta-hemolytic streptococci are further classified by
Lancefield grouping Lancefield grouping is a system of classification that classifies catalase-negative Gram-positive cocci based on the carbohydrate composition of bacterial antigens found on their cell walls. The system, created by Rebecca Lancefield, was historic ...
, a serotype classification (that is, describing specific carbohydrates present on the bacterial cell wall). The 21 described serotypes are named Lancefield groups A to W (excluding I and J). This system of classification was developed by
Rebecca Lancefield Rebecca Craighill Lancefield (January 5, 1895 – March 3, 1981). p.227 was a prominent American microbiologist. She joined the Rockefeller Institute for Medical Research (now Rockefeller University) in New York in 1918, and was associated with ...
, a scientist at Rockefeller University. 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”). Table: Medically relevant streptococci (not all are alpha-hemolytic)


Alpha-hemolytic

When
alpha-hemolysis Hemolysis (from Greek αιμόλυση, meaning 'blood breakdown') is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particularly ...
(α-hemolysis) is present, the agar under the colony will appear dark and greenish due to the conversion of hemoglobin to green
biliverdin Biliverdin (latin for green bile) is a green tetrapyrrolic bile pigment, and is a product of heme catabolism.Boron W, Boulpaep E. Medical Physiology: a cellular and molecular approach, 2005. 984-986. Elsevier Saunders, United States. It is the ...
. ''Streptococcus pneumoniae'' and a group of oral streptococci (''Streptococcus viridans'' or viridans streptococci) display alpha-hemolysis. Alpha-hemolysis is also termed incomplete hemolysis or partial hemolysis because the cell membranes of the red blood cells are left intact. This is also sometimes called green hemolysis because of the color change in the agar.


Pneumococci

* '' S. pneumoniae'' (sometimes called pneumococcus), is a leading cause of bacterial
pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...
and occasional etiology of
otitis media Otitis media is a group of inflammatory diseases of the middle ear. One of the two main types is acute otitis media (AOM), an infection of rapid onset that usually presents with ear pain. In young children this may result in pulling at the ear, ...
,
sinusitis Sinusitis, also known as rhinosinusitis, is inflammation of the mucous membranes that line the sinuses resulting in symptoms that may include thick nasal mucus, a plugged nose, and facial pain. Other signs and symptoms may include fever, head ...
,
meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
, and
peritonitis Peritonitis is inflammation of the localized or generalized peritoneum, the lining of the inner wall of the abdomen and cover of the abdominal organs. Symptoms may include severe pain, swelling of the abdomen, fever, or weight loss. One part or ...
. Inflammation is thought to be the major cause of how pneumococci cause disease, hence the tendency of diagnoses associated with them to involve inflammation. They possess no Lancefield antigens.


The viridans group: alpha-hemolytic

* The
viridans streptococci The viridans streptococci are a large group of commensal streptococcal Gram-positive bacteria species that are α-hemolytic, producing a green coloration on blood agar plates (hence the name "viridans", from Latin "vĭrĭdis", green), although ...
are a large group of
commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
bacteria that are either
alpha-hemolytic Hemolysis (from Greek αιμόλυση, meaning 'blood breakdown') is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particular ...
, producing a green coloration on blood agar plates (hence the name "viridans", from Latin ''vĭrĭdis'', green), or nonhemolytic. They possess no Lancefield antigens.


Beta-hemolytic

Beta-hemolysis Hemolysis (from Greek αιμόλυση, meaning 'blood breakdown') is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particular ...
(β-hemolysis), sometimes called complete
hemolysis Hemolysis or haemolysis (), also known by several other names, is the rupturing (lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo o ...
, is a complete lysis of red cells in the media around and under the colonies: the area appears lightened (yellow) and transparent. Streptolysin, an exotoxin, is the enzyme produced by the bacteria which causes the complete lysis of red blood cells. There are two types of streptolysin: Streptolysin O (SLO) and streptolysin S (SLS). Streptolysin O is an oxygen-sensitive cytotoxin, secreted by most group A ''Streptococcus'' (GAS), and interacts with cholesterol in the membrane of eukaryotic cells (mainly red and white blood cells, macrophages, and platelets), and usually results in beta-hemolysis under the surface of blood agar. Streptolysin S is an oxygen-stable cytotoxin also produced by most GAS strains which results in clearing on the surface of blood agar. SLS affects immune cells, including polymorphonuclear leukocytes and lymphocytes, and is thought to prevent the host immune system from clearing infection. ''Streptococcus pyogenes'', or GAS, displays beta hemolysis. Some weakly beta-hemolytic species cause intense hemolysis when grown together with a strain of ''Staphylococcus''. This is called the CAMP test. ''Streptococcus agalactiae'' displays this property. ''Clostridium perfringens'' can be identified presumptively with this test. ''Listeria monocytogenes'' is also positive on sheep's blood agar.


Group A

Group A ''
S. pyogenes ''Streptococcus pyogenes'' is a species of Gram-positive, aerotolerant bacteria in the genus ''Streptococcus''. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They are ...
'' is the causative agent in a wide range of
group A streptococcal infection Group A streptococcal infections are a number of infections with ''Streptococcus pyogenes'' a group A streptococcus (GAS). ''S. pyogenes'' is a species of beta-hemolytic gram-positive bacteria that is responsible for a wide range of infections t ...
s (GAS). These
infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
s may be noninvasive or invasive. The noninvasive infections tend to be more common and less severe. The most common of these infections include streptococcal pharyngitis (strep throat) and
impetigo Impetigo is a bacterial infection that involves the superficial skin. The most common presentation is yellowish crusts on the face, arms, or legs. Less commonly there may be large blisters which affect the groin or armpits. The lesions may be pa ...
.
Scarlet fever Scarlet fever, also known as Scarlatina, is an infectious disease caused by ''Streptococcus pyogenes'' a Group A streptococcus (GAS). The infection is a type of Group A streptococcal infection (Group A strep). It most commonly affects childr ...
is another example of Group A noninvasive infection. The invasive infections caused by group A beta-hemolytic streptococci tend to be more severe and less common. This occurs when the bacterium is able to infect areas where it is not usually found, such as the
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
and the organs. The diseases that may be caused include streptococcal
toxic shock syndrome Toxic shock syndrome (TSS) is a condition caused by bacterial toxins. Symptoms may include fever, rash, skin peeling, and low blood pressure. There may also be symptoms related to the specific underlying infection such as mastitis, osteomyeli ...
,
necrotizing fasciitis Necrotizing fasciitis (NF), also known as flesh-eating disease, is a bacterial infection that results in the death of parts of the body's soft tissue. It is a severe disease of sudden onset that spreads rapidly. Symptoms usually include red or p ...
,
pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...
, and bacteremia. Globally, GAS has been estimated to cause more than 500,000 deaths every year, making it one of the world's leading
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s. Additional complications may be caused by GAS, namely acute rheumatic fever and acute
glomerulonephritis Glomerulonephritis (GN) is a term used to refer to several kidney diseases (usually affecting both kidneys). Many of the diseases are characterised by inflammation either of the glomeruli or of the small blood vessels in the kidneys, hence the ...
. Rheumatic fever, a disease that affects the
joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...
,
kidneys The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; bloo ...
, and
heart valves A heart valve is a one-way valve that allows blood to flow in one direction through the chambers of the heart. Four valves are usually present in a mammalian heart and together they determine the pathway of blood flow through the heart. A heart v ...
, 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. A similar autoimmune mechanism initiated by Group A beta-hemolytic streptococcal (GABHS) infection is hypothesized to cause pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS), wherein autoimmune antibodies affect the basal ganglia, causing rapid onset of psychiatric, motor, sleep, and other symptoms in pediatric patients. GAS infection is generally diagnosed with a
rapid strep test The rapid strep test (RST) is a rapid antigen detection test (RADT) that is widely used in clinics to assist in the diagnosis of bacterial pharyngitis caused by group A streptococci (GAS), sometimes termed strep throat. There are currently seve ...
or by culture.


Group B

'' S. agalactiae'', or group B ''streptococcus'', GBS, causes pneumonia and meningitis in
newborns An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used t ...
and the elderly, with occasional systemic bacteremia. Importantly, ''Streptococcus agalactiae'' is the most common cause of meningitis in
infant An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
s from one month to three months old. They can also colonize the intestines and the female reproductive tract, increasing the risk for premature
rupture of membranes Rupture of membranes (ROM) or amniorrhexis is a term used during pregnancy to describe a rupture of the amniotic sac. Normally, it occurs spontaneously at full term either during or at the beginning of labor. Rupture of the membranes is known col ...
during pregnancy, and transmission of the organism to the infant. The
American College of Obstetricians and Gynecologists The American College of Obstetricians and Gynecologists (ACOG) is a professional association of physicians specializing in obstetrics and gynecology in the United States. Several Latin American countries are also represented within Districts of ...
,
American Academy of Pediatrics The American Academy of Pediatrics (AAP) is an American professional association of pediatricians, headquartered in Itasca, Illinois. It maintains its Department of Federal Affairs office in Washington, D.C. Background The Academy was founded ...
, and the
Centers for Disease Control The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
recommend all pregnant women between 35 and 37 weeks gestation to be tested for GBS. Women who test positive should be given prophylactic antibiotics during labor, which will usually prevent transmission to the infant. The United Kingdom has chosen to adopt a risk factor-based protocol, rather than the culture-based protocol followed in the US. Current guidelines state that if one or more of the following risk factors is present, then the woman should be treated with ''intrapartum'' antibiotics: * GBS
bacteriuria Bacteriuria is the presence of bacteria in urine. Bacteriuria accompanied by symptoms is a urinary tract infection while that without is known as asymptomatic bacteriuria. Diagnosis is by urinalysis or urine culture. ''Escherichia coli'' is the ...
during this pregnancy * History of GBS disease in a previous infant * Intrapartum fever (≥38 °C) * Preterm labour (<37 weeks) * Prolonged rupture of membranes (>18 hours) This protocol results in the administration of intrapartum antibiotics to 15–20% of pregnant women and prevention of 65–70% of cases of early onset GBS sepsis.


Group C

This group includes ''S. equi'', which causes strangles in horses, and '' S. zooepidemicus''—''S. equi'' is a clonal descendant or
biovar A biovar is a variant prokaryotic strain that differs physiologically or biochemically from other strains in a particular species. Morphovars (or morphotypes) are those strains that differ morphologically. Serovars (or serotypes) are those strai ...
of the ancestral ''S. zooepidemicus''—which causes infections in several species of mammals, including cattle and horses. ''S. dysgalactiae subsp. dysgalactiae'' Note that according to the same source, the subspecies ''equisimilis'' is a grouping of large ''S. dysgalactiae'' colonies, whether they are members of Group C or Group G. is also a member of group C, beta-haemolytic streptococci that can cause
pharyngitis Pharyngitis is inflammation of the back of the throat, known as the pharynx. It typically results in a sore throat and fever. Other symptoms may include a Rhinorrhea, runny nose, cough, headache, difficulty swallowing, swollen lymph nodes, and a ...
and other
pyogenic Pus is an exudate, typically white-yellow, yellow, or yellow-brown, formed at the site of inflammation during bacterial or fungal infection. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection ...
infections similar to
group A streptococci Group A streptococcal infections are a number of infections with ''Streptococcus pyogenes'' a group A streptococcus (GAS). ''S. pyogenes'' is a species of beta-hemolytic gram-positive bacteria that is responsible for a wide range of infections t ...
.


Group D (enterococci)

Many former group D streptococci have been reclassified and placed in the genus ''
Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are gram-positive cocci that often occur in pairs (diplococci) or short chains, and are difficult to distinguish from streptococci on physical char ...
'' (including ''E. faecalis'', ''E. faecium'', ''E. durans'', and ''E. avium''). For example, ''Streptococcus faecalis'' is now ''
Enterococcus faecalis ''Enterococcus faecalis'' – formerly classified as part of the group D ''Streptococcus'' system – is a Gram-positive, commensal bacterium inhabiting the gastrointestinal tracts of humans. Like other species in the genus ''Enterococcus'', ''E ...
''. ''E. faecalis'' is sometimes alpha-hemolytic and ''E. faecium'' is sometimes beta hemolytic. The remaining nonenterococcal group D strains include '' Streptococcus gallolyticus'', ''
Streptococcus bovis ''Streptococcus bovis'' (''S. bovis'') is a species of Gram-positive bacteria that in humans is associated with urinary tract infections, endocarditis, sepsis, Ryan K.J. and C.G. Ray CG (editors). 2004. ''Sherris Medical Microbiology'' (4th ed.). ...
'', '' Streptococcus equinus'' and ''
Streptococcus suis ''Streptococcus suis'' is a peanut-shaped, Gram-positive bacterium, and an important pathogen of pigs. Endemic in nearly all countries with an extensive pig industry, ''S. suis'' is also a zoonotic disease, capable of transmission to humans from ...
''. Nonhemolytic streptococci rarely cause illness. However, weakly hemolytic group D beta-hemolytic streptococci and ''
Listeria monocytogenes ''Listeria monocytogenes'' is the species of pathogenic bacteria that causes the infection listeriosis. It is a facultative anaerobic bacterium, capable of surviving in the presence or absence of oxygen. It can grow and reproduce inside the host ...
'' (which is actually a
gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...
bacillus) should not be confused with nonhemolytic streptococci.


Group F streptococci

Group F streptococci were first described in 1934 by Long and
Bliss BLISS is a system programming language developed at Carnegie Mellon University (CMU) by W. A. Wulf, D. B. Russell, and A. N. Habermann around 1970. It was perhaps the best known system language until C debuted a few years later. Since then, C b ...
amongst the "minute haemolytic streptococci". 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 dysgalactiae subsp. canis'' Note that according to the same source, the subspecies ''equisimilis'' is a grouping of large ''S. dysgalactiae'' colonies, whether they are members of Group C or Group G. is the predominant subspecies encountered. It is a particularly common GGS in humans, although it is typically found on animals. ''S. phocae'' is a GGS subspecies that has been found in marine mammals and marine fish species. In marine mammals it has been mainly associated with
meningoencephalitis Meningoencephalitis (; from ; ; and the medical suffix ''-itis'', "inflammation"), also known as herpes meningoencephalitis, is a medical condition that simultaneously resembles both meningitis, which is an infection or inflammation of the menin ...
,
sepsis Sepsis, formerly known as septicemia (septicaemia in British English) or blood poisoning, is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage is follo ...
, and
endocarditis Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves. Other structures that may be involved include the interventricular septum, the chordae tendineae, the mural endocardium, or the ...
, but is also associated with many other pathologies. Its environmental reservoir and means of transmission in marine mammals is not well characterized.


Group H streptococci

Group H streptococci cause infections in medium-sized canines. Group H streptococci rarely cause human illness unless a human has direct contact with the mouth of a canine. One of the most common ways this can be spread is human-to-canine, mouth-to-mouth contact. However, the canine may lick the human's hand and infection can be spread, as well.


Molecular taxonomy and phylogenetics

Streptococci have been divided into six groups on the basis of their 16S rDNA sequences: ''S. anginosus, S. gallolyticus, S. mitis, S. mutans, S. pyogenes'' and ''S. salivarius''. The 16S groups have been confirmed by whole genome sequencing (see figure). The important pathogens ''S. pneumoniae'' and ''S. pyogenes'' belong to the ''S. mitis'' and ''S. pyogenes'' groups, respectively, while the causative agent of
dental caries Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
, '' Streptococcus mutans'', is basal to the ''Streptococcus'' group. Recent technological advances have resulted in an increase of available genome sequences for ''Streptococcus'' species, allowing for more robust and reliable phylogenetic and comparative genomic analyses to be conducted. In 2018, the evolutionary relationships within ''Streptococcus'' was re-examined by Patel and Gupta through the analysis of comprehensive
phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
s constructed based on four different datasets of proteins and the identification of 134 highly specific molecular signatures (in the form of
conserved signature indels Conserved signature inserts and deletions (CSIs) in protein sequences provide an important category of molecular markers for understanding phylogenetic relationships. CSIs, brought about by rare genetic changes, provide useful phylogenetic markers ...
) that are exclusively shared by the entire genus or its distinct subclades. The results revealed the presence of two main clades at the highest level within ''Streptococcus'', termed the “Mitis-Suis” and “Pyogenes-Equinus-Mutans” clades. The “Mitis-Suis” main clade comprises the Suis subclade and the Mitis clade, which encompasses the Angiosus, Pneumoniae, Gordonii and Parasanguinis subclades. The second main clade, the “Pyogenes-Equinus-Mutans”, includes the Pyogenes, Mutans, Salivarius, Equinus, Sobrinus, Halotolerans, Porci, Entericus and Orisratti subclades. In total, 14 distinct subclades have been identified within the genus ''Streptococcus'', each supported by reliable branching patterns in phylogenetic trees and by the presence of multiple
conserved signature indels Conserved signature inserts and deletions (CSIs) in protein sequences provide an important category of molecular markers for understanding phylogenetic relationships. CSIs, brought about by rare genetic changes, provide useful phylogenetic markers ...
in different proteins that are distinctive characteristics of the members of these 14 clades. A summary diagram showing the overall relationships among the ''Streptococcus'' based on these studies is depicted in a figure on this page.


Genomics

The genomes of hundreds of species have been sequenced. Most ''Streptococcus'' genomes are 1.8 to 2.3 Mb in size and encode 1,700 to 2,300 proteins. Some important genomes are listed in the table. The four species shown in the table (''S. pyogenes, S. agalactiae, S. pneumoniae'', and ''S. mutans'') have an average pairwise protein sequence identity of about 70%.


Bacteriophage

Bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...
s have been described for many species of ''Streptococcus''. 18 prophages have been described in '' S. pneumoniae'' that range in size from 38 to 41 kb in size, encoding from 42 to 66 genes each. Some of the first ''Streptococcus'' phages discovered were Dp-1 and ω1 (alias ω-1).PDF via web archive
(9 Aug 2017)
In 1981 the Cp (Complutense phage 1, officially ''Streptococcus virus Cp1'', '' Picovirinae'') family was discovered with Cp-1 as its first member. Dp-1 and Cp-1 infect both ''S. pneumoniae'' and '' S. mitis''. However, the host ranges of most ''Streptococcus'' phages have not been investigated systematically.


Natural genetic transformation

Natural genetic transformation involves the transfer of DNA from one bacterium to another through the surrounding medium. Transformation is a complex process dependent on expression of numerous genes. To be capable of transformation a bacterium must enter a special physiologic state referred to as competence. ''S. pneumoniae'', ''S. mitis'' and ''S. oralis'' can become competent, and as a result actively acquire homologous DNA for transformation by a predatory fratricidal mechanism This fratricidal mechanism mainly exploits non-competent siblings present in the same niche Among highly competent isolates of ''S. pneumoniae'', Li et al. showed that nasal colonization fitness and virulence (lung infectivity) depend on an intact competence system. Competence may allow the streptococcal pathogen to use external homologous DNA for recombinational repair of DNA damages caused by the hosts oxidative attack.


See also

* Cia-dependent small RNAs *
Quellung reaction The quellung reaction, also called the Neufeld reaction, is a biochemical reaction in which antibodies bind to the bacterial capsule of ''Streptococcus pneumoniae'', ''Klebsiella pneumoniae'', ''Neisseria meningitidis'', '' Bacillus anthracis'', ...
* Streptococcal infection in poultry * Streptococcal pharyngitis *
Streptokinase Streptokinase (SK) is a thrombolytic medication activating plasminogen by nonenzymatic mechanism. As a medication it is used to break down clots in some cases of myocardial infarction (heart attack), pulmonary embolism, and arterial thromboemboli ...


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*
Nature-Inspired CRISPR Enzyme Discoveries Vastly Expand Genome Editing
On: SciTechDaily. June 16, 2020. Source: Media Lab, Massachusetts Institute of Technology.
Streptococcus
genomes and related information a
PATRIC
a Bioinformatics Resource Center funded b
NIAID



The UK Group B Strep Support
charity {{Authority control Streptococcaceae Gut flora bacteria Gram-positive bacteria Bacteria genera Pathogenic bacteria