Streptococcus anginosus- An Overview

Streptococcus anginosus is a Gram-positive bacterium that belongs to the anginosus group of viridans streptococci, which also includes Streptococcus intermedius and Streptococcus constellatus. These bacteria are part of the normal human flora that colonize the mouth, upper respiratory tract, gastrointestinal tract, and vagina. However, they can also cause various infections of the oral and non-oral sites, especially in immunocompromised individuals or those with underlying conditions.

S. anginosus was first isolated and identified by Andrewes and Horder in 1906 from the human gut. The species name ‘anginosus‘ is taken from the term ‘angina’, indicating the association between the species and the disease angina, causing chest pain due to lack of oxygen to the muscles. S. anginosus is sometimes also referred to as Streptococcus milleri, but this name is now pseudotaxonomic as it was based on the erroneous assumption that all members of the anginosus group belonged to a single species.

S. anginosus is a facultative anaerobe that can grow in air with 5% carbon dioxide or under anaerobic conditions. It can produce acetoin from glucose, ferment lactose, trehalose, salicin, and sucrose, and hydrolyze esculin and arginine. It can also produce hydrogen sulfide (H2S) from cysteine, which is a unique feature among streptococci and contributes to its pathogenicity. S. anginosus may be alpha-hemolytic, beta-hemolytic, or non-hemolytic on blood agar, and some strains may produce a distinct butterscotch or caramel odor.

S. anginosus has several virulence factors that enable it to adhere to host cells, evade phagocytosis, destroy extracellular matrix components, and form abscesses. These include adhesins, capsule, H2S production, and hydrolytic enzymes. S. anginosus is associated with various infections such as bacteremia, endocarditis, brain abscesses, liver abscesses, pulmonary infections, intra-abdominal infections, osteomyelitis, prosthetic joint infections, and soft tissue infections. Some studies have also suggested a link between S. anginosus and colorectal carcinoma or abscesses.

The diagnosis of S. anginosus infections is based on the isolation and identification of the organism from clinical specimens using morphological, cultural, biochemical, and molecular methods. The treatment of S. anginosus infections depends on the site and severity of infection, but penicillin, ampicillin, erythromycin, and tetracycline are usually effective against most strains. In some cases, surgical drainage or removal of infected tissue may be required.

The species of the genus Streptococcus have been classified on the basis of their DNA-DNA hybridization and rRNA sequences into about 60 different species and 12 subspecies.

The genus belongs to the family Streptococcaceae which is a part of the low (<50%) G+C content branch of Eubacteria based on their rRNA sequences.

Streptococcus species are closely related to the group of lactic acid bacteria as they produce lactic acid as their primary end product during carbon metabolism.

S. anginosus belongs to the anginosus group of Streptococci, which in itself has been the subject of much taxonomic confusion, partly due to lack of international consensus on terminology and partly due to lack of reliable distinguishing phenotypic markers.

Although the term ‘Streptococcus milleri group’ is still widely used, the term ‘Anginosus group’ proposed by Kawamura is preferred as it may eliminate much of the nomenclatural confusion.

The group consists of two other species, namely; S. intermedius and S. constellatus, which were previously considered the subspecies of S. anginosus but were later realized as separate species based on their molecular characteristics.

Some strains of S. anginosus belong to group F of the Lancefield antigen group while the rest are non-groupable.

The following is the taxonomical classification of S. anginosus:

• Streptococcus anginosus is part of the bacterial normal flora in humans where it colonizes a wide variety of parts within the human body .
• The primary site of colonization of the organism is the oral cavity, gastrointestinal tract, and vaginal tract .
• Besides, they have also been isolated from other areas of the body like the hepatic sites, upper respiratory tract, and urinary tract .
• S. anginosus is harmless as a commensal but can cause various infections if it makes its way into the sterile sites of the body.
• The natural habitat or source of the bacteria is not yet known; however, it is assumed that it is transmitted to humans soon after birth via the environment and the primary care-giver.
• The organism has a different mechanism to survive in such parts of the human body and reduce its competition.
• The average temperature for the growth and survival of the organism is the average human body temperature, but it can survive in the temperature range of 30-40°C.

The cells of S. anginosus are Gram-positive, round, spherical, or cocci that are small with about 0.5–1.0 µm average diameter. The cells are arranged in short chains with longer chains seen when grown on liquid culture media. The arrangement of the cells is characteristic of the genus Streptococci with successive division planes that are parallel resulting in cells arranged in the form of chains.

The organism is catalase-negative, facultative anaerobe, not capsulated, and usually carries sparsely distributed, long fibrils on the surface.

The cell wall is made up of peptidoglycan, teichoic acid, and different types of carbohydrates. The peptidoglycan type found in S. anginosus is Lys-Ala1-3.

The teichoic acid present in the cell wall is ribitol teichoic acid and Rhamnose, Glucose and Galactose are the major cell-wall carbohydrate constituents.

The peptidoglycan in the cell wall is a long chain of glycans that are cross-linked through short peptides, and the individual glycan is made of β-1,4-linked units alternating with N-acetylglucosamine and N-acetylmuramic acid.

An outer capsular polysaccharide is also present on S. anginosus that is involved in the pathogenesis of the infections caused by the organism.

A lipid-protein bilayer is present underneath the cell wall that functions as a cell membrane. It contains various transport chains that help in the movement of molecules between the cell and the environment.

Streptococcus anginosus does not show profuse growth on traditional media like Nutrient Agar and might require supplements like carbohydrates and other nutrients. Media like Blood agar and Chocolate agar are commonly used for the observation of the pattern of hemolysis that is important during the identification of S. anginosus. For more selective isolation, media like Brain Heart Infusion Agar and Trypticase soy agar/broth with defibrinated sheep blood can be used. It is a facultative anaerobe, so abundant growth is observed in the air with 5% carbon dioxide at 37°C. The growth is reduced under aerobic conditions, is frequently enhanced by the addition of CO2 with some strains requiring anaerobic incubation. No growth is seen at 10°C, but growth is positive at 45°C. The organism can tolerate 40% bile but cannot tolerate 6.5% NaCl and pH 9.6.

• Nutrient Agar: White to grey colored colonies of an average size of 1 mm in diameter. The colonies were round with raised elevation and an entire margin. Growth is mostly poor and requires air with supplied carbon dioxide.
• Blood Agar: Colonies on blood (horse) agar are 0.5–2.0 mm in diameter, white or translucent, convex, and entire. Some strains might produce colonies that are white, and matte with an average diameter of 0.5–1.0 mm. Most strains give α-hemolysis or no hemolysis on blood agar with some strains giving β-hemolysis . Pronounced greening is observed on chocolate agar.
  
  Biochemical characteristics of Streptococcus anginosus
  

The biochemical characteristics of S. anginosus can be used to differentiate it from other members of the anginosus group and other streptococci. The biochemical tests are based on the fermentation of different carbohydrates and the production of different enzymes by the bacteria. The following table summarizes some of the common biochemical tests and their results for S. anginosus :

S. anginosus can also produce acetoin from glucose, which can be detected by the Voges-Proskauer test. Most strains of S. anginosus are non-hemolytic or alpha-hemolytic on blood agar, but some strains can produce beta-hemolysis. The colonies of S. anginosus often have a distinct odor of butterscotch or caramel.

Streptococcus anginosus is a commensal bacterium that colonizes the oral cavity, gastrointestinal tract, and vagina of humans. However, it can also cause various infections, especially abscesses, in different parts of the body when it breaches the mucosal barriers. The virulence factors of S. anginosus are not well understood, but some of them have been identified and characterized. These include:

• Adhesins: These are surface proteins that mediate the attachment of S. anginosus to the host cells and extracellular matrix proteins. S. anginosus produces a fibronectin-binding protein that is involved in the adhesion to the heart valves in infective endocarditis. It also produces a laminin-binding protein that binds to the exposed basement membrane of the mucosal tissues. Moreover, S. anginosus expresses homologs of other streptococcal adhesins, such as pneumococcal surface adhesin A (PsaA) and pullulanase (PulA), which have high affinity to human plasmin. The enolase enzyme also acts as a plasminogen-binding protein on the bacterial surface.
• Capsule: S. anginosus has a polysaccharide capsule that surrounds the cell wall and protects it from phagocytosis and phagocytic killing by the host immune cells. The capsule also helps in biofilm formation and evasion of neutrophil extracellular traps. The gene locus for capsule synthesis in S. anginosus is similar to that of S. pneumoniae.
• Hydrogen sulfide (H2S): S. anginosus can produce H2S from L-cysteine by the enzyme L-cysteine desulfhydrase. H2S is a toxic gas that can interact with hemoglobin and cause erythrocyte lysis, methemoglobinemia, and sulfhemoglobinemia. H2S also contributes to abscess formation by creating an anaerobic environment and enhancing bacterial growth.
• Hydrolytic enzymes: S. anginosus secretes various enzymes that degrade nucleic acids or components of the extracellular matrix, such as hyaluronic acid and chondroitin sulfate. These enzymes include nucleases, hyaluronidase, chondroitin sulphatase, sialidase, beta-galactosidase, N-acetyl-beta-glucosaminidase, and N-acetyl-beta-galactosaminidase. These enzymes help in spreading the infection through the tissue and escaping from the immune defense.

These virulence factors enable S. anginosus to colonize, invade, damage, and evade the host tissues and cause various infections ranging from dental caries to endocarditis. Further studies are needed to elucidate the molecular mechanisms and regulation of these factors and their role in pathogenesis.

Traditionally, S. anginosus has been considered non-pathogenic, but new studies have indicated that S. anginosus is involved in various infections throughout various parts of the body. The exact mechanism of infection is not yet clearly understood but the involvement of some structures and proteins that are homologous to that found in other pathogenic Streptococcal species.

1. Colonization
   Streptococcus anginosus is a commensal that reaches different parts of the body via different sources after birth. The organism primarily colonizes the gastrointestinal tract where it adheres to the epithelial cells of the mucosal membrane. Adhesion is the first step in the pathogenesis of the infection, which is brought about by different structures and proteins produced by the bacteria. The binding of bacteria to the extracellular matrix proteins is a common mechanism of streptococci that has also been observed in S. anginosus. S. anginosus codes for both fibrinogen-binding and laminin-binding proteins that bind to the extracellular matrix of the host cells. The attachment is further enhanced by surface adhesins that are homologous to the pneumococcal surface Adhesin A (PsaA) of S. pneumoniae and the pullulanase PulA of S. pyogenes. Besides, the enzyme enolase produced by the bacteria helps in the colonization of the epithelial surface.

2. Invasion
   Colonization of the epithelial surface is then followed by invasion, which is supported by various enzymes that help in disrupting the outer barrier. S. anginosus is surrounded by capsular polysaccharide that inhibits phagocytosis and phagocytic killing and allows the bacteria to divide. The l‐cysteine desulfhydrase of S. anginosus generates H2S from l‐cysteine, resulting in an increased abscesses formation by an interaction with hemoglobin. Exposure of erythrocytes to H2S leads to the production of sulfhaemoglobin, methemoglobin, and erythrocyte lysis. S. anginosus also produces hydrolytic enzymes that destroy nucleic acids or components of the extracellular matrix-like hyaluronic acid and chondroitin sulfate and aids in the destruction of the mucosal layer. The hydrolytic enzyme nucleases play an important role in immune evasion of neutrophil extracellular traps, while hyaluronidase and chondroitin sulphatase act as spreading factors by the digestion of extracellular matrix. The bacteria then move through the bloodstream and reach different parts of the body like the heart valves. In some cases, S. anginosus induces inflammation reaction and causes the release of IL-8 which, in turn, causes the release of carcinogens that might initiate malignancy.

   Clinical manifestations of Streptococcus anginosus

Streptococcus anginosus can cause various infections in different parts of the body, ranging from mild to severe. The clinical manifestations depend on the site and extent of infection, as well as the underlying conditions of the patient. Some of the common clinical manifestations are:

• Infections of the oropharynx: These include pharyngitis, tonsillitis, peritonsillar abscess, and retropharyngeal abscess. Patients may present with sore throat, odynophagia, dysphagia, fever, cervical lymphadenopathy, trismus, and neck pain . In some cases, the infection may spread to the head and neck regions, causing otitis media, sinusitis, mastoiditis, and deep neck space infections.
• Infections of the respiratory tract: These include pneumonia, lung abscess, empyema, and bronchitis. Patients may have cough, sputum production, dyspnea, chest pain, fever, and pleural effusion . S. anginosus is often associated with aspiration pneumonia in patients with impaired swallowing or altered consciousness.
• Infections of the abdominal cavity: These include intra-abdominal abscess, peritonitis, appendicitis, cholecystitis, and liver abscess. Patients may have abdominal pain, fever, vomiting, diarrhea, jaundice, and signs of peritoneal irritation . S. anginosus has been linked to colorectal carcinoma and malignancy-associated abscesses in some cases.
• Infections of the bloodstream: These include bacteremia and septicemia. Patients may have fever, chills, malaise, hypotension, and systemic toxemia . S. anginosus bacteremia is often secondary to a primary focus of infection in the oropharynx or abdomen.
• Infections of the heart: These include endocarditis and myocarditis. Patients may have fever, heart murmur, chest pain, dyspnea, peripheral emboli, and heart failure . S. anginosus is a common cause of infective endocarditis among viridans streptococci and can affect both native and prosthetic valves.
• Infections of the soft tissue and bones: These include cellulitis, necrotizing fasciitis, myositis, osteomyelitis, septic arthritis, and epidural abscess. Patients may have pain, swelling, erythema, warmth, purulent discharge, and reduced mobility at the affected site . S. anginosus can cause severe soft tissue infections that require surgical debridement.
• Infections of the central nervous system: These include meningitis, brain abscess, subdural empyema, and Lemierre syndrome. Patients may have headache, fever, neck stiffness, altered mental status, focal neurological deficits, cranial nerve palsies, and thrombophlebitis . S. anginosus can cause intracranial infections as a complication of oropharyngeal or sinus infections.

The diagnosis and treatment of S. anginosus infections require appropriate microbiological tests and antimicrobial therapy. In some cases, surgical drainage or removal of infected foci may be necessary.

Lab diagnosis of Streptococcus anginosus

The diagnosis of infections caused by S. anginosus begins with the collection of samples, which depends on the site of infection and its extent. The sample can either be observed directly under the microscope or in other cases, cultured to obtain viable colonies of the organism for further diagnosis. The diagnosis of diseases caused by S. anginosus is mostly involved with the identification of the organism.

1. Morphological, cultural, and biochemical characteristics

The observation of the organism under the microscope, followed by its culture and biochemical tests is the most common method during the diagnosis of S. anginosus.

• Isolation of the organism from primary clinical specimens is obtained on selective culture media like blood agar supplemented with 5 percent sheep blood, following an incubation period of 18–24 h in the air at 35–37°C .
• Some strains might require anaerobic incubation while others grow well in air supplemented with about 5% carbon dioxide.
• Colony morphology and hemolysis can then provide a basis for biochemical testing for species-level identification.
• Different biochemical tests beginning with catalase and oxidase can provide a better distinction of the species.
• The following table summarizes some of the biochemical characteristics of S. anginosus:

2. Rapid diagnosis systems

Nowadays, different techniques utilize different properties of the species like the fatty acid profile and surface antigens for rapid identification of the organism.

• For Gram-positive organisms, different test kits like MicroScan Conventional Pos ID, Rapid Pos ID, and BBL Crystal Gram-Pos ID are available.
• These tests, however, might not very reliable and thus should only be used as a primary identification method followed by identification through culture or molecular methods.

3. Molecular methods

Molecular methods of diagnosis are based on the identification of the organism based on its nuclear sequences.

• This is the most accurate method of identification as the gene sequences of an organism are mostly unique.
• In molecular methods, techniques that study the DNA sequences and rRNA sequences are used, some of which are PCR and ribotyping.
• These methods use restriction fragment polymorphism technique and DNA-DNA hybridization.
• Some of the genes that have been used for molecular identification of S. anginosus are 16S rRNA gene, sodA gene, groEL gene, recN gene, and rpoB gene.

Treatment of Streptococcus anginosus

The treatment of infections caused by S. anginosus depends on the site and severity of the infection, the susceptibility of the organism, and the underlying condition of the patient. Some general principles of treatment are:

• Penicillin, ampicillin, and erythromycin are the drugs of choice for most infections caused by S. anginosus as these bacteria are rarely resistant to penicillin .
• In cases of penicillin allergy, clindamycin, tetracycline, or vancomycin can be used as alternative agents.
• In cases of endocarditis, a combination of penicillin and gentamicin is recommended for at least four weeks.
• In cases of abscesses, surgical drainage or aspiration may be required in addition to antimicrobial therapy.
• In cases of bacteremia, blood cultures should be repeated until they become negative to ensure clearance of the infection.
• In cases of association with colorectal carcinoma or hepatobiliary disease, screening and treatment of the underlying condition may be necessary.

The optimal duration of therapy for S. anginosus infections is not well established and may vary depending on the clinical response and the type of infection. However, some general guidelines are:

• For uncomplicated skin and soft tissue infections, 7 to 10 days of therapy may be sufficient.
• For complicated skin and soft tissue infections, osteomyelitis, septic arthritis, or meningitis, 4 to 6 weeks of therapy may be required.
• For intra-abdominal abscesses or peritonitis, 10 to 14 days of therapy may be adequate.
• For endocarditis or prosthetic joint infection, 6 weeks or longer of therapy may be necessary.

The treatment of S. anginosus infections should be guided by the results of culture and susceptibility testing whenever possible. The antimicrobial susceptibility patterns of S. anginosus may vary depending on the geographic region and the source of isolation. Some strains may show resistance to cephalosporins, macrolides, tetracyclines, or trimethoprim-sulfamethoxazole . Therefore, empirical therapy should be based on local epidemiology and modified according to culture results.

S. anginosus infections can be prevented by maintaining good oral hygiene, avoiding contact with infected animals or contaminated pork products, and seeking prompt medical attention for any signs or symptoms of infection. Patients with risk factors for endocarditis (such as prosthetic heart valves or congenital heart defects) should receive prophylactic antibiotics before dental or surgical procedures that may cause bacteremia.

The diagnosis of infections caused by S. anginosus begins with the collection of samples, which depends on the site of infection and its extent. The sample can either be observed directly under the microscope or in other cases, cultured to obtain viable colonies of the organism for further diagnosis. The diagnosis of diseases caused by S. anginosus is mostly involved with the identification of the organism.

1. Morphological, cultural, and biochemical characteristics

The observation of the organism under the microscope, followed by its culture and biochemical tests is the most common method during the diagnosis of S. anginosus.

• Isolation of the organism from primary clinical specimens is obtained on selective culture media like blood agar supplemented with 5 percent sheep blood, following an incubation period of 18–24 h in the air at 35–37°C .
• Some strains might require anaerobic incubation while others grow well in air supplemented with about 5% carbon dioxide.
• Colony morphology and hemolysis can then provide a basis for biochemical testing for species-level identification.
• Different biochemical tests beginning with catalase and oxidase can provide a better distinction of the species.
• The following table summarizes some of the biochemical characteristics of S. anginosus:

2. Rapid diagnosis systems

Nowadays, different techniques utilize different properties of the species like the fatty acid profile and surface antigens for rapid identification of the organism.

• For Gram-positive organisms, different test kits like MicroScan Conventional Pos ID, Rapid Pos ID, and BBL Crystal Gram-Pos ID are available.
• These tests, however, might not very reliable and thus should only be used as a primary identification method followed by identification through culture or molecular methods.

3. Molecular methods

Molecular methods of diagnosis are based on the identification of the organism based on its nuclear sequences.

• This is the most accurate method of identification as the gene sequences of an organism are mostly unique.
• In molecular methods, techniques that study the DNA sequences and rRNA sequences are used, some of which are PCR and ribotyping.
• These methods use restriction fragment polymorphism technique and DNA-DNA hybridization.
• Some of the genes that have been used for molecular identification of S. anginosus are 16S rRNA gene, sodA gene, groEL gene, recN gene, and rpoB gene.

The treatment of infections caused by S. anginosus depends on the site and severity of the infection, the susceptibility of the organism, and the underlying condition of the patient. Some general principles of treatment are:

• Penicillin, ampicillin, and erythromycin are the drugs of choice for most infections caused by S. anginosus as these bacteria are rarely resistant to penicillin .
• In cases of penicillin allergy, clindamycin, tetracycline, or vancomycin can be used as alternative agents.
• In cases of endocarditis, a combination of penicillin and gentamicin is recommended for at least four weeks.
• In cases of abscesses, surgical drainage or aspiration may be required in addition to antimicrobial therapy.
• In cases of bacteremia, blood cultures should be repeated until they become negative to ensure clearance of the infection.
• In cases of association with colorectal carcinoma or hepatobiliary disease, screening and treatment of the underlying condition may be necessary.

The optimal duration of therapy for S. anginosus infections is not well established and may vary depending on the clinical response and the type of infection. However, some general guidelines are:

• For uncomplicated skin and soft tissue infections, 7 to 10 days of therapy may be sufficient.
• For complicated skin and soft tissue infections, osteomyelitis, septic arthritis, or meningitis, 4 to 6 weeks of therapy may be required.
• For intra-abdominal abscesses or peritonitis, 10 to 14 days of therapy may be adequate.
• For endocarditis or prosthetic joint infection, 6 weeks or longer of therapy may be necessary.

The treatment of S. anginosus infections should be guided by the results of culture and susceptibility testing whenever possible. The antimicrobial susceptibility patterns of S. anginosus may vary depending on the geographic region and the source of isolation. Some strains may show resistance to cephalosporins, macrolides, tetracyclines, or trimethoprim-sulfamethoxazole . Therefore, empirical therapy should be based on local epidemiology and modified according to culture results.

S. anginosus infections can be prevented by maintaining good oral hygiene, avoiding contact with infected animals or contaminated pork products, and seeking prompt medical attention for any signs or symptoms of infection. Patients with risk factors for endocarditis (such as prosthetic heart valves or congenital heart defects) should receive prophylactic antibiotics before dental or surgical procedures that may cause bacteremia.

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