In addition to
streptococcal pharyngitis (strep throat), certain
Streptococcus species are responsible for many cases of
pink eye,
meningitis,
bacterial pneumonia,
endocarditis,
erysipelas, and
necrotizing fasciitis (the 'flesh-eating' bacterial infections). 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 ("Swiss") 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 (that is, describing specific carbohydrates present on the bacterial cell wall). 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 (also known as "group A strep" and "group B strep").
Table: Medically relevant streptococci Beta-hemolytic Beta-hemolysis (β-hemolysis), sometimes called complete
hemolysis, 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.
(right) and beta-hemolytic S. pyogenes'' (left) streptococci growing on blood agar
Group A Group A
S. pyogenes is the causative agent in a wide range of
group A streptococcal infections (GAS). 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 (strep throat) 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. 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 (≥38 °C) • Preterm labour (18 hours) This protocol results in the administration of intrapartum antibiotics to 15–20% of pregnant women and the 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 of the ancestral
S. zooepidemicus — which causes infections in several species of mammals, including cattle and horses.
S. dysgalactiae subsp. dysgalactiae is also a member of group C,
beta-haemolytic streptococci that can cause
pharyngitis and other
pyogenic infections similar to
group A streptococci. Group C streptococcal bacteria are considered zoonotic pathogens, meaning infection can be passed from animal to human.
Group D (enterococci) Many former group D streptococci have been reclassified and placed in the genus
Enterococcus (including
E. faecalis,
E. faecium,
E. durans, and
E. avium). For example,
Streptococcus faecalis is now
Enterococcus faecalis.
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 equinus and
Streptococcus suis. 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 among 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 == Clinical identification==