Streptococcus

Streptococcus (plural, Streptococci) is a type of bacteria. The name refers to a group of related bacteria called a genus; individual types are species. Many species of Streptococcus are common and do not cause any human disease. Some species, often under specific conditions, can cause a variety of infections. Often these are mild, but others are severe.

Streptococci are a diverse genus, infecting both humans and animals. Their importance has grown recently as multiple antibiotic-resistant strains have appeared and caused disease outbreaks.

Streptococcus is Gram positive, which is a broad categorization of bacteria based on how they react to a certain type of staining material.

Other Names

• Strep

Among the most commonly-encountered terms referring to certain types of Streptococcus are:

• Strep. pneumo or pneumococcus
• Viridans strep or Strep. viridans
• Group A beta-hemolytic streptococcus, also called Strep pyogenes, group A strep, GABHS, or GAS
• Group B strep, also called GBS

Types

Group A strep (S. pyogenes) colonies showing beta-hemolysis. Source: CDC

Classification schemes

Streptococcus is a broad genus that includes many species. Streptococcus species are usually divided by whether they can break open (or lyse) red blood cells—a process known as hemolysis. There are three kinds of hemolysis: alpha (α), beta (β), and gamma (γ). Hemolysis is normally detected by growing bacteria on blood agar, a nutrient that contains whole red blood cells. β-hemolysis is complete rupture of red blood cells, giving distinct, wide, clear areas around bacterial colonies on blood agar. α-hemolysis is less complete and appears as a greenish color on blood agar. Other streptococci are labeled as γ-hemolytic for the sake of consistency, even though they cause no hemolysis.

In human infections, the four most important types of strep are:

1. α-hemolytic streptococci :

a. Streptococcus pneumoniae, a single species, also known as the pneumococcus.

b. Streptococcus Viridans group, a group of species that behave similarly.

2. β-hemolytic streptococci:

a. Group A strep (GAS or GABHS), for the most part a single species called Streptococcus pyogenes.

b. Group B strep (GBS), for the most part a single species called Streptococcus agalactiae.

α-hemolytic species

The most commonly encountered α-hemolytic Streptoccus species is Streptococcus pneumoniae, usually called the pneumococcus, a leading cause of bacterial pneumonia, ear infections (otitis media), and, in rare cases, meningitis.

The other main type of α-hemolytic strep is called the Streptococcus Viridans-group. It includes the following species.

• Streptococcus viridans, a cause of endocarditis and dental abscesses
• Streptococcus mutans, a contributor to tooth decay (dental caries)
• Streptococcus mitis, mostly found around cheek region
• Streptococcus sanguinis, no preference of locations
• Streptococcus salivarius, mostly found on the top of the tongue
• Streptococcus thermophilus, used in the manufacture of some cheeses and yogurts
• Streptococcus constellatus, which occasionally causes human disease

β-hemolytic species

β-hemolytic streptococci are further classified into groups A through O based on the type of carbohydrate on their surface. These are called Lancefield groups after Dr. Rebecca Lancefield who developed the classification scheme in the early part of the 20th century. The grouping is based on antibodies' binding to the bacteria (serology). Groups A and B strep are the most important for human disease.

Group A Streptococcus (often abbreviated to GAS or GABHS) are primarily of the species Streptococcus pyogenes, which causes strep throat and other infections, including acute rheumatic fever, scarlet fever, acute glomerulonephritis and necrotizing fasciitis. Other Streptococcus species may also possess the Group A antigen, but human infections by these are uncommon.

Group B Streptococcus (abbreviated to GBS) are primarily of the species Streptococcus agalactiae. GBS can cause pneumonia, sepsis and meningitis, especially in newborns and the elderly. They can also live in the intestines and the female reproductive tract, increasing the risk for early breaking of water during pregnancy (premature rupture of membranes) and transmission of the infection 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 of pregnancy should be tested for GBS. Women who test positive should be given preventive antibiotics during labor, which will usually prevent transmission to the infant.^[1]

Effect on Humans

Streptococci are also part of the normal bacteria that live in the mouth, skin, intestine, and upper respiratory tract of humans. Most streptococcal species do not normally cause disease. When they do cause disease, strep infections include:

• Strep Throat
• Impetigo
• Cellulitis and erysipelas
• Scarlet fever

They also include severe, life-threatening strep infections such as:

• Sepsis (bacteria in the blood)
• Puerperal fever
• Toxic shock syndrome
• Necrotizing fasciitis (commonly called flesh-eating disease)
• Myositis
• Meningitis
• Endocarditis

The most common strep infection is strep throat.

Two diseases that can occur after a Strep pyogenes infection has subsided are acute rheumatic fever and post-streptococcal glomerulonephritis. These are not infections, but diseases that result from previous infections. See Complications.

Microbiology

Streptococci are Gram-positive bacteria, which means that when they are stained by the Gram stain they take up the dye and look blue under the microscope. (Gram-negative bacteria look red.) Streptococci are nonmotile, meaning they do not move about on their own. They live in pairs or chains of varying length. Unlike some other Gram-positive bacteria, they do not form spores (tough cells resistant to harsh environments). They are characteristically round or oval in shape. Many Streptococci produce a capsule of linked sugars as a coating. These capsules protect the bacteria from the host's immune system.

Identification and diagnosis

Streptococcal identification and diagnosis depends on the site of infection and the nature of the symptoms.

Treatment

Streptococcal infections need to be treated with antibiotics. Penicillin is frequently used.

• Strep throat can be treated with a single injection of penicillin G or penicillin V orally for 10 days. Amoxicillin or cephalosporins, related drugs, can also be used. Erythromycin and the related drugs azithromycin and clarithromycin can be used in patients with an allergy to penicillin.

• [[Impetigo]]: Treated with penicillin, retapamulin, and a number of other medications that also kill Staph aureus (a common cause of impetigo), such as cephalexin, amoxicillin and clavulanate, or clindamycin. In mild cases, mupirocin ointment may be used on the skin.

• Cellulitis and erysipelas: Treated with penicillin, dicloxacillin, nafcillin, cephalexin, doxycycline, or amoxicillin and clavulanate. Daptomycin or vancomycin are used in severe cases.

• Pyoderma, or pus-producing infections in the skin, can often be caused by either strep or another gram-positive bacterium called Staphylococcus aureus. For this reason—and because doctors seldom know at first which of these bacteria is the culprit—such infections should be treated with an antibiotic that eradicates both types of bacteria. Examples include cloxacillin, cephalexin, or cefaclor. Now that infections with MRSA are very common, antibiotics that treat it are usually the best choice; these should be combined with a drug like penicillin that treats strep. These include trimethoprim/sulfamethoxazole, clindamycin, doxycycline, or vancomycin in severe cases.

• Scarlet fever: Treated with penicillin or erythromycin.

• Sepsis (bacteria in the blood), meningitis, and endocarditis: Treated with penicillin, ampicillin, vancomycin, and occasionally other drugs.

• Streptococcal toxic shock syndrome and necrotizing fasciitis (sometimes called "flesh-eating disease"): Treated with penicillin. Clindamycin may be added to penicillin, since very large numbers of bacteria may overwhelm penicillin's effectiveness.

Prevention

There are several vaccines available to prevent the alpha-hemolytic Streptococcus pneumoniae (or the pneumococcus). The so-called pneumococcal conjugate vaccines and pneumococcal polysaccharide vaccines are effective in preventing childhood and adult diseases caused by the pneumococcus, including the ear infection otitis media, pneumonia, and meningitis.

For beta-hemolytic strep, vaccines for GABHS and GBS are under development. See Research.

Complications of GABHS

GABHS is notorious for its many complications. There are two types of complications that can arise: suppurative (involving pus) and nonsuppurative.

Suppurative complications

Suppurative complications of GABHS infections include:^[2]

• Bacteremia, or bacteria in the bloodstream
• Cervical lymphadenitis, or infected lymph nodes in the neck
• Endocarditis, or bacteria growing on heart valves
• Fasciitis/myositis syndrome, or infections of the muscles and their compartments
• Mastoiditis, or infection of the bone behind the ear
• Meningitis
• Otitis media, an ear infection
• Perianal dermatitis, or irritation around the anus in children
• Peritonsillar abscess or retropharyngeal abscess: pockets of pus in the back of the throat
• Pneumonia
• Sinusitis
• Toxic shock syndrome

Nonsuppurative complications

Nonsuppurative complications of GABHS infections are those that happen because of an abnormal immune-system reaction to the bacterium. GABHS in sore throats and scarlet fever tends to break into molecular pieces that resemble certain molecules that occur naturally in the body. This molecular mimicry confuses the immune system, which attacks not only the strep but also those molecules in the body, such as heart valves.^[2]

Nonsuppurative complications can usually be prevented by, but not treated with, antibiotics. They include the following illnesses.^[2]

• Poststreptococcal glomerulonephritis is an inflammation of the kidneys that may follow an untreated strep throat but more often comes after a strep skin infection.
• Poststreptococcal reactive arthritis
• Rheumatic fever can develop two to three weeks after a bout of strep throat and can lead to arthritis and damage to heart valves.^[3]
• [[[Sydenham_Chorea|Sydenham chorea]] is a disorder in which the muscles of the torso, arms, and legs move involuntarily in a dancing and jerky manner.
• Pediatric autoimmune neuropsychiatric disorders associated with streptococcal infection (PANDAS)
• Poststreptococcal autoimmune dystonia secondary to striatal necrosis, a type of brain damage^[4]

Research

Group A strep

Through research, health experts have learned that there are more than 120 different strains of group A streptococci (GAS) bacteria, each producing its own unique proteins. Some of these proteins are responsible for specific GAS diseases.

With the support of the National Insitute of Allergy and Infectious Diseases (NIAID), scientists have determined the genetic sequence, or DNA code, for five strains of GAS. By studying its genes, scientists can learn which proteins are responsible for virulence, which is crucial information that will lead to new and improved drugs and vaccines.

NIAID supports research to develop a GAS vaccine. Several possible vaccines are in various phases of development. While some scientists are conducting animal model studies to obtain data to pursue clinical trials in humans, other scientists are close to evaluating GAS vaccine candidates in Phase I clinical trials.

As a result of NIAID-supported research, the first GAS vaccine clinical trial in 30 years was started. The vaccine was well-tolerated by patients and has led to further clinical evaluation of a similar vaccine candidate.

An effective vaccine will prevent not only strep throat and impetigo but also more serious invasive disease and post-infectious complications like rheumatic fever.

Vaccine development efforts include NIAID-supported epidemiological studies:

• To determine the extent of GAS disease
• To characterize GAS strains causing illness in the United States and developing countries

NIAID, in collaboration with the Centers for Disease Control and Prevention and the World Health Organization, has developed standard definitions and methods for surveillance of GAS diseases.

Group B strep

References

1. ↑ Schrag S, Gorwitz R, Fultz-Butts K, Schuchat A. Prevention of perinatal group B streptococcal disease. Revised guidelines from CDC. MMWR Recomm Rep. 2002 Aug 16;51(RR-11):1-22. Abstract
2. ↑ ^{2.0 2.1 2.2} Hahn RG, Knox LM, Forman TA. Evaluation of poststreptococcal illness. Am Fam Physician. 2005 May 15;71(10):1949-54. Abstract | Full Text
3. ↑ Cilliers AM. Rheumatic fever and its management. BMJ. 2006 Dec 2;333(7579):1153-6. Abstract | Full Text
4. ↑ Dale RC, Church AJ, Benton S, Surtees RA, Lees A, Thompson EJ, Giovannoni G, Neville BG. Post-streptococcal autoimmune dystonia with isolated bilateral striatal necrosis. Dev Med Child Neurol. 2002 Jul;44(7):485-9. Abstract

External Links

Medical college of Wisconsin: Strep Throat

CDC: Group A Streptococcal Disease