Streptococcus 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 that tend to link in chains. They are clinically important for humans, as they are an infrequent, but usually pathogenic, part of the skin microbiota that can cause group A streptococcal infection. S. pyogenes is the predominant species harboring the Lancefield group A antigen, and is often called group A Streptococcus. However, both Streptococcus dysgalactiae and the Streptococcus anginosus group can possess group A antigen as well. Group A streptococci, when grown on blood agar, typically produce small zones of beta-hemolysis, a complete destruction of red blood cells. The name group A Streptococcus is thus also used.
The species name is derived from Greek words meaning 'a chain' of berries and pus -forming, since a number of infections caused by the bacterium produce pus. The main criterion for differentiation between Staphylococcus spp. and Streptococcus spp. is the catalase test. Staphylococci are catalase positive whereas streptococci are catalase-negative. S. pyogenes can be cultured on fresh blood agar plates. The PYR test allows for the differentiation of Streptococcus pyogenes from other morphologically similar beta-hemolytic streptococci as S. pyogenes will produce a positive test result.
An estimated 700 million GAS infections occur worldwide each year. While the overall mortality rate for these infections is less than 0.1%, over 650,000 of the cases are severe and invasive, and these cases have a mortality rate of 25%. Early recognition and treatment are critical; diagnostic failure can result in sepsis and death. S. pyogenes is clinically and historically significant as the cause of scarlet fever, which results from exposure to the species' exotoxin.
Epidemiology
Unlike most bacterial pathogens, S. pyogenes only infects humans. Thus, zoonotic transmission from an animal to a human is rare.S. pyogenes typically colonizes the throat, genital mucosa, rectum, and skin. Of healthy adults, 1% to 5% have throat, vaginal, or rectal carriage, with children being more common carriers. Most frequently, transmission from one person to another occurs due to inhalation of respiratory droplets, produced by sneezing and coughing from an infected person. Skin contact, contact with objects harboring the bacterium, and consumption of contaminated food are possible but uncommon modes of transmission. Streptococcal pharyngitis occurs most frequently in late winter to early spring in most countries as indoor spaces are used more often and thus more crowded. Disease cases are the lowest during autumn.
Maternal S. pyogenes infection usually happens in late pregnancy, at more than 30 weeks of gestation to four weeks postpartum. Maternal infections account for 2 to 4% of all clinically diagnosed S. pyogenes infections. The risk of sepsis is relatively high compared to other bacterial infections acquired during pregnancy, and S. pyogenes is a leading cause of septic shock and death in pregnant and postpartum women.
Bacteriology
Serotyping
In 1928, Rebecca Lancefield published a method for serotyping S. pyogenes based on its cell-wall polysaccharide, a virulence factor displayed on its surface. Later, in 1946, Lancefield described the serologic classification of S. pyogenes isolates based on components of their surface pili which are used by bacteria to attach to host cells. As of 2016, a total of 120 M proteins have been identified. These M proteins are encoded by 234 type emm genes with greater than 1,200 alleles.Lysogeny
All strains of S. pyogenes are polylysogenized, in that they carry one or more bacteriophage in their genomes. Some of the phages may be defective, but in some cases active phage may compensate for defects in others. In general, the genome of S. pyogenes strains isolated during disease are >90% identical, they differ by the phage they carry.Virulence factors
S. pyogenes has several virulence factors that enable it to attach to host tissues, evade the immune response, and spread by penetrating host tissue layers. A carbohydrate-based bacterial capsule composed of hyaluronic acid surrounds the bacterium, protecting it from phagocytosis by neutrophils. In addition, the capsule and several factors embedded in the cell wall, including M protein, lipoteichoic acid, and protein F facilitate attachment to various host cells. M protein also inhibits opsonization by the alternative complement pathway by binding to host complement regulators. The M protein found on some serotypes is also able to prevent opsonization by binding to fibrinogen. However, the M protein is also the weakest point in this pathogen's defense, as antibodies produced by the immune system against M protein target the bacteria for engulfment by phagocytes. M proteins are unique to each strain, and identification can be used clinically to confirm the strain causing an infection.| Name | Description |
| Streptolysin O | An exotoxin, one of the bases of the organism's beta-hemolytic property, streptolysin O causes an immune response and detection of antibodies to it; antistreptolysin O can be clinically used to confirm a recent infection. It is damaged by oxygen. |
| Streptolysin S | A cardiotoxic exotoxin, another beta-hemolytic component, not immunogenic and O2 stable: A potent cell poison affecting many types of cell including neutrophils, platelets, and subcellular organelles. |
| Streptococcal pyrogenic exotoxin A | Superantigens secreted by many strains of S. pyogenes: This streptococcal pyrogenic exotoxin is responsible for the rash of scarlet fever and many of the symptoms of streptococcal toxic shock syndrome, also known as toxic shock like syndrome. |
| Streptococcal pyrogenic exotoxin C | Superantigens secreted by many strains of S. pyogenes: This streptococcal pyrogenic exotoxin is responsible for the rash of scarlet fever and many of the symptoms of streptococcal toxic shock syndrome, also known as toxic shock like syndrome. |
| Streptococcal pyrogenic exotoxin B | A cysteine protease and the predominant secreted protein. Multiple actions, including degrading the extracellular matrix, cytokines, complement components, and immunoglobulins. Also called streptopain. |
| Streptokinase | Enzymatically activates plasminogen, a proteolytic enzyme, into plasmin, which in turn digests fibrin and other proteins |
| Hyaluronidase | Hyaluronidase is widely assumed to facilitate the spread of the bacteria through tissues by breaking down hyaluronic acid, an important component of connective tissue. However, very few isolates of S. pyogenes are capable of secreting active hyaluronidase due to mutations in the gene that encodes the enzyme. Moreover, the few isolates capable of secreting hyaluronidase do not appear to need it to spread through tissues or to cause skin lesions. Thus, the true role of hyaluronidase in pathogenesis, if any, remains unknown. |
| Streptodornase | Most strains of S. pyogenes secrete up to four different DNases, which are sometimes called streptodornase. The DNases protect the bacteria from being trapped in neutrophil extracellular traps by digesting the NETs' web of DNA, to which are bound neutrophil serine proteases that can kill the bacteria. |
| C5a peptidase | C5a peptidase cleaves a potent neutrophil chemotaxin called C5a, which is produced by the complement system. C5a peptidase is necessary to minimize the influx of neutrophils early in infection as the bacteria are attempting to colonize the host's tissue. C5a peptidase, although required to degrade the neutrophil chemotaxin C5a in the early stages of infection, is not required for S. pyogenes to prevent the influx of neutrophils as the bacteria spread through the fascia. |
| Streptococcal chemokine protease | The affected tissue of patients with severe cases of necrotizing fasciitis are devoid of neutrophils. The serine protease ScpC, which is released by S. pyogenes, is responsible for preventing the migration of neutrophils to the spreading infection. ScpC degrades the chemokine IL-8, which would otherwise attract neutrophils to the site of infection. |