Streptococcus pyogenes
Streptococcus pyogenes is a species of Gram-positive, aerotolerant bacterium in the genus Streptococcus. These bacteria are extracellular, and made up of non-motile and non-sporing cocci. It is clinically important for humans. It is an infrequent, but usually pathogenic, part of the skin microbiota. It 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. Group A streptococci when grown on blood agar typically produces small zones of beta-hemolysis, a complete destruction of red blood cells. It is thus also called group A Streptococcus, and it can make colonies greater than 5 mm in size.
Like other cocci, streptococci are round bacteria. The species name is derived from Greek words meaning 'a chain' of berries and pus -forming, because streptococcal cells tend to link in chains of round cells and 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. Under ideal conditions, it has an incubation period of 1 to 3 days.
An estimated 700 million GAS infections occur worldwide each year. While the overall mortality rate for these infections is 0.1%, over 650,000 of the cases are severe and invasive, and have a mortality rate of 25%. Early recognition and treatment are critical; diagnostic failure can result in sepsis and death.
Epidemiology
S. pyogenes typically colonizes the throat, genital mucosa, rectum, and skin. Of healthy individuals, 1% to 5% have throat, vaginal, or rectal carriage. In healthy children, such carriage rate varies from 2% to 17%. There are four methods for the transmission of this bacterium: inhalation of respiratory droplets, skin contact, contact with objects, surface, or dust that is contaminated with bacteria or, less commonly, transmission through food. Such bacteria can cause a variety of diseases such as streptococcal pharyngitis, rheumatic fever, rheumatic heart disease, and scarlet fever. Although pharyngitis is mostly viral in origin, about 15 to 30% of all pharyngitis cases in children are caused by GAS; meanwhile, 5 to 20% of pharyngitis in adults are streptococcal. The number of pharyngitis cases is higher in children when compared with adults due to exposures in schools, nurseries, and as a consequence of lower host immunity. Such cases Streptococcal pharyngitis occurs more frequently from December to April in seasonal countries, possibly due to changing climate, behavioural changes or predisposing viral infection. Disease cases are the lowest during autumn.MT1 clone is frequently associated with invasive Streptococcus pyogenes infections among developed countries. The incidence and mortality of S. pyogenes was high during the pre-penicillin era, but had already started to fall prior to the widespread availability of penicillin. Therefore, environmental factors do play a role in the S. pyogenes infection. Incidence of S. pyogenes is 2 to 4 per 100,000 population in developed countries and 12 to 83 per 100,000 population in developing countries. S. pyogenes infection is more frequently found in men than women, with highest rates in the elderly, followed by infants. In people with risk factors such as heart disease, diabetes, malignancy, blunt trauma, surgical incision, virus respiratory infection, including influenza, S. pyogenes infection happens in 17 to 25% of all cases. GAS secondary infection usually happens within one week of the diagnosis of influenza infection. In 14 to 16% of childhood S. pyogenes infections, there is a prior chickenpox infection. Such S. pyogenes infection in children usually manifests as severe soft tissue infection with onset 4 to 12 days from the chickenpox diagnosis. There is also 40 to 60 times increase in risk of S. pyogenes infection within the first two weeks of chickenpox infection in children. However, 20 to 30% of S. pyogenes infection does occur in adults with no identifiable risk factors. The incidence is higher in children with no known risk factors. The rates of scarlet fever in UK was usually 4 in 100,000 population, however, in 2014, the rates had risen to 49 per 100,000 population. Rheumatic fever and rheumatic heart disease usually occurs at 2 to 3 weeks after the throat infection, which is more common among the impoverished people in developing countries. From 1967 to 1996, the global mean incidence of rheumatic fever and RHD was 19 per 100,000 with the highest incidence at 51 per 100,000.
Maternal S. pyogenes infection usually happens in late pregnancy; at more than 30 weeks of gestation to four weeks postpartum, which accounts for 2 to 4% of all the S. pyogenes infections. This represents 20 to 100 times increase in risk for S. pyogenes infections. Clinical manifestations are: pneumonia, septic arthritis, necrotizing fasciitis, and genital tract sepsis. According to a study done by Queen Charlotte's hospital in London during the 1930's, the vagina was not the common source of such infection. On the contrary, maternal throat infection and close contacts with carriers were the more common sites for maternal S. pyogenes infection.
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 their surface T-antigen. Four of the 20 T-antigens have been revealed to be pili, which are used by bacteria to attach to host cells. As of 2016, a total of 120 M proteins are identified. These M proteins are encoded by 234 types emm gene with greater than 1,200 alleles.Lysogeny
All strains of S. pyogenes are polylysogenized, in that they carry one or more bacteriophage on 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 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 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. |
Genome
The genomes of different strains were sequenced encoding about 1700-1900 proteins. Complete genome sequences of the type strain of S. pyogenes are available in DNA Data Bank of Japan, European Nucleotide Archive, and GenBank under the accession numbers and .Biofilm formation
Biofilms are a way for S. pyogenes, as well as other bacterial cells, to communicate with each other. In the biofilm gene expression for multiple purposes is controlled via quorum sensing.One of the biofilm forming pathways in GAS is the Rgg2/3 pathway. It regulates SHP's that are quorum sensing pheromones a.k.a. autoinducers. The SHP's are translated to an immature form of the pheromone and must undergo processing, first by a metalloprotease enzyme inside the cell and then in the extracellular space, to reach their mature active form. The mode of transportation out of the cell and the extracellular processing factor are still unknown. The mature SHP pheromone can then be taken into nearby cells and the cell it originated from via a transmembrane protein, oligopeptide permease.
In the cytosol the pheromones have two functions in the Rgg2/3 pathway. Firstly, they inhibit the activity of Rgg3 which is a transcriptional regulator repressing SHP production. Secondly, they bind another transcriptional regulator, Rgg2, that increases the production of SHP's, having an antagonistic effect to Rgg3. SHP's activating their own transcriptional activator creates a positive feedback loop, which is common for the production for quorum sensing peptides. It enables the rapid production of the pheromones in large quantities. The production of SHP's increases biofilm biogenesis.
It has been suggested that GAS switches between biofilm formation and degradation by utilizing pathways with opposing effects. Whilst Rgg2/3 pathway increases biofilm, the RopB pathway disrupts it. RopB is another Rgg-like protein that directly activates SpeB, a cysteine protease that acts as a virulence factor. In the absence of this pathway, biofilm formation is enhanced, possibly due to the lack of the protease degrading pheromones or other Rgg2/3 pathway counteracting effects.
Disease
S. pyogenes is the cause of many human diseases, ranging from mild superficial skin infections to life-threatening systemic diseases. Infections typically begin in the throat or skin. The most striking sign is a strawberry-like rash. Examples of mild S. pyogenes infections include pharyngitis and localized skin infection. Erysipelas and cellulitis are characterized by multiplication and lateral spread of S. pyogenes in deep layers of the skin. S. pyogenes invasion and multiplication in the fascia can lead to necrotizing fasciitis, a life-threatening condition requiring surgery. The bacterium is found in neonatal infections.Infections due to certain strains of S. pyogenes can be associated with the release of bacterial toxins. Throat infections associated with release of certain toxins lead to scarlet fever. Other toxigenic S. pyogenes infections may lead to streptococcal toxic shock syndrome, which can be life-threatening.
S. pyogenes can also cause disease in the form of post-infectious "non-pyogenic" syndromes. These autoimmune-mediated complications follow a small percentage of infections and include rheumatic fever and acute post-infectious glomerulonephritis. Both conditions appear several weeks following the initial streptococcal infection. Rheumatic fever is characterized by inflammation of the joints and/or heart following an episode of streptococcal pharyngitis. Acute glomerulonephritis, inflammation of the renal glomerulus, can follow streptococcal pharyngitis or skin infection.
This bacterium remains acutely sensitive to penicillin. Failure of treatment with penicillin is generally attributed to other local commensal organisms producing β-lactamase, or failure to achieve adequate tissue levels in the pharynx. Certain strains have developed resistance to macrolides, tetracyclines, and clindamycin.