Streptococcus pyogenes

Morphology

The individual cocci of streptococcus pyogenes are spherical or oval having size of 0.5 – 1.0 micrometer in diameter. This size depends upon the cultural conditions, for example, when grown anaerobically, they are somewhat smaller.

They are arranged in chains the length of which varies within wide limits and is influenced by nature of the culture medium, these chains being longer in liquid than in solid media.

Chain formation in streptococcus is due to the cocci dividing in one plane only and the daughter cells failing to separate completely.

Some non pathogenic streptococci form the longest chains, for example, Str. salivarius.

Streptococci are nonmotile and non sporing. But some strains of Str. pyogenes and some group C strains have capsules composed of haluronic acid, while polysaccharide capsules are encountered in members of group B and D. these capsules are best seen in young culture of streptococci.

Cultural Characteristics

Streptococci in general an aerobe and a facultative anaerobe. They grow best at temperature of 37 C (range 22 – 42 C). it is exacting in nutritive requirements, growth occurring only in media containing fermentable carbohydrates or enriched with blood or serum.

On blood agar, after incubation for 24 hours, the colonies are small (0.5 – 1.0 mm) circular, semitransparent, low convex discs with an area of clear hemolysis around them. There growth and hemolysis are promoted by 10% CO2.

Virulent strains, on fresh isolation from lesions, produce a ‘matt’ (finely granular) colony, while a virulent strains form ‘glossy’ colonies.

Very rarely, nonhemolytic group A streptococci are encountered, which are typical of Str. pyogenes in other respects.

In liquid media, such as glucose or serum broth, growth occurs as a granular turbidity with a powdery deposit.

Biochemical Reactions

Streptococci ferment several sugars producing acid but not gas.

Streptococci are catalase negative. They are not voluble in 10% bile, unlike pneumococci.

Str. pyogenes can be identify among other str. as they are performing hydrolysis of pyrrolidonly naphthylamide (PYR test) and failure to ferment ribose.

Resistance

Str. pyogenes is a delicate organism, easily destroyed by heat (54 C for 30 minutes). It dies in a few days in culture, unless stored at a low temperature (4 C), preferably in Robertson’s cooked meat medium. If protected from sunlight, it can survive in dust for several weeks. It is rapidly inactivated by antiseptics. It is more resistant to crustal violet than many bacteria.

Toxins And Other Virulence Factors

Str. pyogenes forms several exotoxins and enzymes which contribute to its virulence.

The M protein also acts as a virulence factor by inhibiting phagocytosis. The C polysaccharide has been shown to have a toxic effect on connective tissue in experimental animals.

Hemolysins

Streptococci produce two hemolysins, streptomycin ‘O’ and ‘S’. streptomycin O is so called because it is oxygen labile. It appears to be important in contributing to virulence. It is lethal on intravenous injection into animals and has a specific cardio toxic activity. It has leucotoxic activity also.

Pyogenic Exotoxin (Erythrogenic, Dick, Scarlatinal Toxin)

This toxin was named ‘erythrogenic’ because its in trader al injection into susceptible individuals produced an erythematous reaction (Dick test, 1924). This test was used to identify children susceptible to scarlet fever, a type of acute pharyngitis with extensive erythematous rash, caused by the Str. Pyogenes strains producing this toxin. Blanching of the rash on local injection of convalescent serum was used as a diagnostic, test for scarlet fever.

The primary effect of the toxin is induction of fever and so it was renamed streptococcal pyogenic exotoxin (SPE).

There are three type of SPE have been identified – SPE A, B and C. type A and C are coded for by bacteriophage genes, while type B gene is chromosomal. SPEs are ‘superantigen’, T cell mitogenes which induce massive release of inflammatory cytokines causeing fever, shock and tissue damage.

Pathogenicity

Str. pyogenes produces pyogenic infections with a tendency to spread locally, along lymphatics and through the bloodstream.

Respiratory Infections

The primary site of invasion of the human body by str. pyogenes is the throat. Sore throat is the most common of streptococcal diseases. It may be localised as tonsillitis or may involve the pharynx more diffusely (pharyngitis).

From the throat, streptococci may spread to the surrounding tissues, leading to suppurative complications, such as otitis media, mastoiditis, quinsy. It may rarely leads to meningitis. Streptococcal pneumonia seldom follows throat infection but may occur as a complication of influenza or other respiratory viral diseases.

Skin And Soft Tissue Infections

Str. pyogenes causes a variety of suppurative infections of the skin, including infection of wounds or burns, with a predilection to produce lymphangitis and cellulitis. Infection of minor abrasion may at times lead to fatal septicemia.

The two typical streptococcal infections of the skin are crysipelas and impetigo. The former is a diffuse infection involving the superficial lymphatics. The affected skin, which is red, swollen and infuriated, is sharply demarcated from the surrounding healthy area.

Genital Infections

Both aerobic and anaerobic streptococci are normal inhabitants of the female genitalia. Str. pyogenes was an important cause of puerperal sepsis, with the infection usually being exogenous. Puerperal fever is now much more commonly due to endogenous infection with anaerobic streptococci.

Other Suppurative Infections

Str. pyogenes may cause abscesses in internal organs such as the brain, lungs, liver and kidneys,and also septicemia and premia.

Laboratory Diagnosis

In acute infection diagnosis is established by culture, while in the non suppurative complications, diagnosis is mainly based on the demonstration of antibodies.

For cultures, swabs should be collected under vision from the affected site and either plated immediately or sent to the laboratory in Pike’s medium. The specimen is plated on blood agar and incubated at 37 C anaerobically or number 5 – 10 % CO2, as hemolysis develops better under these conditions.

Sheep blood agar is recommended for primary isolation because it is inhibitory for Haemophilus haemolyticus, colonies of which may be confused with those of hemolytic streptococci.

Treatment

All beta hemolytic group A streptococci are sensitive to penicillin G, and most are sensitive to erythromycin. In patients allergic to penicillin, erythromycin or cephalexin may be used. Strains resistant to erythromycin have been reported. Tetracyclines and sulphonamides are not recommended. Antimicrobial drugs have no effect on established glomerulonephritis and rheumatic fever.

Reference:

Text Book Of Microbiology

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine

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