Streptococcus pyogenes- Group A Streptococcus (GAS)- An Overview

They are also known as Group A Streptococcus (GAS) or Group A (beta-hemolytic) Streptococcus (GABHS).

Habitat of Streptococcus pyogenes

1. They are found in the throat and skin of humans.
2. They are commensals of the upper respiratory tract.
3. The Carrier rate of S. pyogenes in the respiratory tract is only few in adults, over 10% in children attending kindergarten.
4. Opportunistic pathogens
5. May survive in dust for some time.

Morphology of Streptococcus pyogenes

1. Gram +ve bacteria
2. Spherical or oval cocci in chains.
3. 5 to 1 μm in diameter.
4. Non-motile
5. Non-sporing
6. Some of the strains are capsulated, best seen in young cultures.
7. The cell wall contains the important component, group-specific carbohydrates.
8. Fimbriated
9. It has a protein called Protein F.

Genome of Streptococcus pyogenes

1. Base pair: 1,852,442
2. Protein encoding genes: 1752
3. G+C Content: 38.5%

Cultural Characteristics of Streptococcus pyogenes

• Like most Streptococcus species, Streptococcus pyogenes also don’t grow well on a simple medium with basic nutrients. The growth, however, occurs well on agar media supplemented with blood.
• The use of blood agar facilitates the observation of β-hemolysis, which differentiates S. pyogenes from other Streptococcus species.
• S. pyogenes isolated from oral swabs grow well on sucrose-containing agar medium like Trypticase Yeast Extract Cystine with 5% sucrose.
• Other selective media for S. pyogenes include the agar medium like Columbia agar with colistin commonly used for the culture of Gram-positive bacteria.
• In the case of clinical samples, ample growth of typical S. pyogenes typical colonies can be seen after 24 hours at 35-37°C.
• The incubation of blood agar plates in anaerobic or CO2-rich environment often leads to the isolation of non-S. pyogenes β-hemolytic streptococci.
• Strep Selective agar is best for the suppression of commensal respiratory microbiota, including the species of the same genus.
•  The optimum temperature for the growth of S. pyogenes is at 37°C, but growth can be seen between 15°C to 40°C.
• The inability of the bacteria to grow at 10°c and 45°C as well as at 6.5% NaCl and 40% bile helps differentiate S. pyogenes from other Streptococcus species.
• The bacteria is an aerobic or facultatively anaerobic bacteria that can tolerate comparatively higher levels of oxygen and can also grow at a low level of oxygen. About 5-10% CO2 during incubation promotes hemolysis on blood agar.
• S. pyogenes also grows well in liquid culture media like Nutrient agar and Glucose broth. The growth is observed in the form of granular turbidity with a powdery deposit as a result of heavy bacterial chains that settle down and form powdery deposits.

Figure: Streptococcus pyogenes inoculated on trypticase soy agar containing 5% defibrinated sheep’s blood. Image Source: CDC/ Richard R. Facklam, Ph.D.

The following are some cultural characteristics of S. pyogenes on different culture media:

1. Nutrient Agar

• The colonies of S. pyogenes on NA appear circular pinpoint with an average diameter of 0.5-1 mm.
• The colonies are light yellow to yellow colored semi-transparent to opaque with low convex or convex elevation with matt surface (in the case of virulent strains) or glossy (in the case of non-virulent strains) and mucoid (in the case of capsule producing strains).

2. Blood Agar

• On blood agar, S. pyogenes form circular pinpoint colonies that are similar in morphology to the colonies formed on other solid agar media.
• Light golden yellow colonies are formed that are surrounded by a clear zone exhibiting β-hemolysis.
• The surface of the colonies differs in different species based on their virulence and production of the capsule.

3. PNF medium

• Circular pinpoint colonies of S. pyogenes are observed on PNF medium that are yellow-colored.
• Like on blood agar, S. pyogenes also produces β-hemolysis around the colonies on the PNF medium.

Virulence factors of Streptococcus pyogenes

A. Antigenic structure

• M protein: rod-like coiled structure with two major structural classes; Class I and Class II; major virulence factor; resist phagocytosis and intracellular killing by polymorphonuclear leukocytes in the absence of antibodies.

Figure: Virulence factors of Streptococcus pyogenes. Image Source: Christian Linke-Winnebeck.

B. Toxins and enzymes

1. Streptokinase

• It is also called fibrinolysin.
• It transforms the plasminogen of human plasma into plasmin, an active proteolytic enzyme that digests fibrin and other proteins, allowing the bacteria to escape from blood clots.

2. Deoxyribonucleases

• Streptococcal deoxyribonucleases A, B, C, and D degrades DNA (DNases) and similar to streptokinase facilitate the spread of streptococci in tissue by liquefying pus.

3. Hyaluronidase

• Hyaluronidase splits hyaluronic acid, an important component of the ground substance of connective tissue. Thus, hyaluronidase aids in spreading infecting microorganisms (spreading factor).

4. Pyrogenic exotoxins (Erythrogenic toxins)

• It acts as superantigens, which stimulate T cells by binding to class II MHC complex, and the activated T cell release cytokines that mediate shock and tissue injury.
• It is associated with Streptococcal toxic shock syndrome and scarlet fever.

5. Hemolysins

• Two hemolysins are produced.
• Streptolysin O is oxygen labile and immunogenic in nature. It induces the production of Anti-Streptolysin O(ASO) after the infection with streptococci.
• Streptolysin S is oxygen stable and not immunogenic in nature. It s an agent responsible for the hemolytic zone around the streptococcal colonies on the surface of blood agar.

Clinical manifestation of Streptococcus pyogenes

Disease attributable to invasion by S. pyogenes

1. Erysipelas

• The Portal of entry is skin
• Raised lesion and red
• Brawny edema

2. Cellulitis

• Acute
• Spreading infection of the skin and subcutaneous tissue
• Pain, tenderness, swelling, and erythema

3. Necrotizing fasciitis

• Rapidly spreading necrosis of skin tissue and fascia

4. Puerperal fever

• If the organism enters the uterus after delivery, puerperal fever develops
• Septicemia

5. Bacteremia and sepsis

• Infection of traumatic or surgical wounds with streptococci results in bacteremia, which can rapidly be fatal.

Figure: Pathogenesis of Streptococcus pyogenes infections. Image Source: Kenneth Todar.

Disease attributable to local infection with S. pyogenes and their byproduct

1. Streptococcal sore throat

• Subacute nasopharyngitis
• Thin serous discharge
• Fever
• Infection extends to the middle ear or mastoid
• Enlarged cervical lymph nodes
• Tonsillitis
• Intense redness and edema of mucous membranes
• Purulent exudates

2. Streptococcal pyoderma

• Infection of the superficial layer of skin: impetigo
• Superficial vesicles
• Denuded surface covered with pus and later encrusted

Invasive Group A Streptococcal infection

1. Streptococcal toxic shock syndrome and scarlet fever

• Shock
• Bacteremia
• Respiratory failure
• Multiorgan failure
• Necrotizing fasciitis
• Myositis
• Fever
• Erythema and desquamation

Poststreptococcal disease

1. Acute glomerulonephritis

• Blood and protein in the urine
• Edema
• High blood pressure
• Urea nitrogen retention
• Low serum complement levels
• Chronic form leads to kidney failure

2. Rheumatic fever

• Most serious sequela
• Damage to heart muscles and valves
• Fever
• Malaise
• A migratory non-suppurative polyarthritis
• Inflammation of all parts of the heart ( endocardium, myocardium, and pericardium)
• Thickened and deformed valves.

Laboratory diagnosis of Streptococcus pyogenes

1. Specimens

• Throat swab
• Pus
• Cerebrospinal fluid
• Blood
• Serum for antibody determinant

2. Smear

• Gram staining
• Purple color cocci in a chain arrangement
• Not to be confused with Viridans Streptococci from throat swab sample since both have the same appearance.

Figure: Photomicrograph of a specimen revealing numbers of chain-linked Streptococcus pyogenes bacteria. Image Source: CDC.

3. Culture

• Culture on blood agar
• Addition of bacitracin in inoculum: S pyogenes are sensitive to bacitracin
• Colonial appearance: Grayish white, transparent to translucent, matte or glossy; smooth; flat; large zone of beta hemolysis
• Catalase negative, oxidase negative, and PYR positive.

4. Antigen detection tests

• Enzyme immunoassay (EIA)
• Agglutination test
• Kits use enzymatic or chemical method to extract antigen from a throat swab and demonstrate the presence of antigen using EIA or agglutination test (visible clumping)
• More sensitive assays are DNA probes and Nucleic acid amplification techniques

5. Serologic tests

• Detection of antibody titer after 3 to 4 weeks after exposure to the organism
• Antibodies include ASO, anti-DNase B, anti- hyaluronidase, antistreptokinase, anti- M type-specific antibodies
• Anti Streptolysin O (ASO) is most widely used.

Treatment of Streptococcus pyogenes infections

• S pyogenes are susceptible to penicillin (benzylpenicillin (penicillin G) or oral phenoxymethylpenicillin (penicillin V).
• For penicillin-allergic patients, erythromycin is the drug of choice.
• In some cases, clindamycin or vancomycin is also recommended.

Prevention and control of Streptococcus pyogenes infections

• Maintenance of personal hygiene
• Chemoprophylaxis: prophylactic use of antibiotics in some streptococcal infections: rheumatic fever.

References and Source