Streptococcus
Streptococcus, from Ancient Greek στρεπτός, meaning "twisted", and κόκκος, meaning "grain", is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales, in the phylum Bacillota. Cell division in streptococci occurs along a single axis, thus when growing they tend to form pairs or chains, which may appear bent or twisted. This differs from staphylococci, 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.
The term was coined in 1877 by Viennese surgeon Albert Theodor Billroth, from Ancient Greek στρεπτός, meaning "twisted", and κόκκος, meaning "grain". In 1984, many bacteria formerly grouped in the genus Streptococcus were separated out into the genera Enterococcus and Lactococcus. 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, certain Streptococcus species are responsible for many cases of pink eye, meningitis, bacterial pneumonia, endocarditis, erysipelas, and necrotizing fasciitis. However, many streptococcal species are not pathogenic, and form part of the commensal human microbiota of the mouth, skin, intestine, and upper respiratory tract. Streptococci are also a necessary ingredient in producing Emmentaler cheese.Species of streptococci are classified based on their hemolytic properties. Alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells, giving it a greenish color on blood agar. Beta-hemolytic 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, a serotype classification. The 21 described serotypes are named Lancefield groups A to W. This system of classification was developed by Rebecca Lancefield, a scientist at Rockefeller University.
In the medical setting, the most important groups are the alpha-hemolytic streptococci S. pneumoniae and Streptococcus ''viridans groups, and the beta-hemolytic streptococci of Lancefield groups A and B.
Table: Medically relevant streptococci
| Species | Host | Disease |
| S. pyogenes | human | pharyngitis, cellulitis, erysipelas |
| S. agalactiae | human, cattle | neonatal meningitis and sepsis |
| S. dysgalactiae | human, animals | endocarditis, bacteremia, pneumonia, meningitis, respiratory infections |
| S. gallolyticus | human, animals | biliary or urinary tract infections, endocarditis |
| S. anginosus | human, animals | subcutaneous/organ abscesses, meningitis, respiratory infections |
| S. sanguinis | human | endocarditis, dental caries |
| S. suis | swine | meningitis |
| S. mitis | human | endocarditis |
| S. mutans | human | dental caries |
| S. pneumoniae'' | human | pneumonia |
Alpha-hemolytic
When alpha-hemolysis is present, a blood based agar under the colony will appear dark and greenish due to the conversion of hemoglobin to green biliverdin. Streptococcus pneumoniae and a group of oral 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, is a leading cause of bacterial pneumonia and the occasional etiology of otitis media, sinusitis, meningitis, and peritonitis. 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 are a large group of commensal bacteria that are either alpha-hemolytic, producing a green coloration on blood agar plates, or nonhemolytic. They possess no Lancefield antigens.
Beta-hemolytic
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.
File:Alpha and Beta haemolytic streptococci.jpg|thumb|250px|Alpha-hemolytic S. viridans and beta-hemolytic S. pyogenes streptococci growing on blood agar
Group A
Group A S. pyogenes is the causative agent in a wide range of group A streptococcal infections. These infections 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 and impetigo. Scarlet fever 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 and organs. The diseases that may be caused include streptococcal toxic shock syndrome, necrotizing fasciitis, pneumonia, and bacteremia. Globally, GAS has been estimated to cause more than 500,000 deaths every year, making it one of the world's leading pathogens.
Additional complications may be caused by GAS, namely acute rheumatic fever and acute glomerulonephritis. 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, but due to 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 infection is hypothesized to cause pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections, 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 or by culture.
Group B
S. agalactiae, or group B streptococcus, GBS, causes pneumonia and meningitis in newborns and the elderly, with occasional systemic bacteremia. Importantly, Streptococcus agalactiae is the most common cause of meningitis in infants 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 during pregnancy, and transmission of the organism 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 to be tested for GBS. Women who test positive should be given prophylactic antibiotics during labor, which will usually prevent transmission to the infant. Group III polysaccharide vaccines have been proven effective in preventing the passing of GBS from mother to 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 during this pregnancy
- History of GBS disease in a previous infant
- Intrapartum fever
- Preterm labour
- Prolonged rupture of membranes