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In this article we will discuss about Streptococci:- 1. Meaning of Streptococci 2. Morphology and Staining of Streptococci 3. Cultural Characteristics 4. Biochemical Reaction 5. Antigenic Structure 6. Toxins and Enzymes 7. Haemolysin 8. Infective Endocarditis 9. Sub-Acute Endocarditis 10. Post-Streptococcal Disease 11. Laboratory Diagnosis 12. Serological Tests 13. Treatment.
Contents:
- Meaning of Streptococci
- Morphology and Staining of Streptococci
- Cultural Characteristics of Streptococci
- Biochemical Reaction of Streptococci
- Antigenic Structure of Streptococci
- Toxins and Enzymes Produced by Streptococci
- Haemolysin
- Infective Endocarditis
- Sub-Acute Endocarditis
- Post-Streptococcal Disease
- Laboratory Diagnosis of Streptococci
- Serological Tests of Streptococci
- Treatment of Streptococci
1. Meaning of Streptococci:
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The streptococci are Gram-positive spherical or oval bacteria that form short or long chains. They are associated with important human diseases (tonsillitis, respiratory infection, skin infection, genital, suppurative and non-suppurative infections (rheumatic fever, glomerulonephritis).
2. Morphology and Staining of Streptococci:
Individual cocci are spherical (0.5-1 µ in diameter) and arranged in chains (Fig. 24.1). The lengths of chains are variable. The chains are longer in liquid than in solid media. Streptococci are Gram-positive. As the culture ages, they loose their Gram- positivity and appear to be Gram-negative.
They are non-motile non-sporulating. Some strain of group A and C have capsules composed of hyaluronic acid. Hair-like pili project through the capsule of group A Streptococci and are important in the attachment of streptococci to epithelial cells.
3. Cultural Characteristics of Streptococci:
They grow on solid media as discoid colonies—usually 1-2 mm in diameter. Group A strains that produce capsular material often given rise to mucoid colonies. They are aerobic; and facultative anaerobic grow best at 37°C. They grow only in media enriched with blood or serum.
On blood agar, after 24 hours incubation at 10% CO2, colonies are small (0.5-1 mm), circular, semi-transparent, low convex discs with an area of clear haemolysis around them.
Virulent strains, strains isolated from lesions produce a “matt” (finest granular) colony, while avirulent strains form “glossy” colonies. In liquid media (glucose or serum broth) growth occurs as a granular turbidity with a powdery deposit. No pellicle is formed.
4. Biochemical Reaction of Streptococci:
Streptococci ferment several sugars producing an acid but no gas. Fermentation of sugars has been replaced by serological grouping. They are catalase negative and not soluble in 10% bile, unlike pneumococci.
5. Antigenic Structure of Streptococci:
Streptococci are grouped into alpha (Str. viridans), beta (Str. pyogenes), and gamma (Str. faecalis) haemolytic streptococci : Haemolytic streptococci can be classified into 20 serological groups (A-V except I, J) on the basis of the nature of carbohydrate ‘C’ antigen.
The majority of haemolytic streptococci that produce human infection belong to group A—known as Str. pyogenes. Streptococci from primary isolation plates can be grouped by a recent Reverse Passive Haemagglutination (RPHA) technique.
6. Toxins and Enzymes Produced by Streptococci:
More than 20 extra-cellular products that are produced by group A streptococci include streptokinase, streptodornase, hyaluronidase, erythrogenic toxin, diphosphopyridine nucleotides, haemolysin.
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Erythrogenic Toxin:
It is soluble and destroyed by boiling for 1 hour. It causes rash in scarlet fever. Only lysogenic strains elaborating this toxin cause scarlet fever. This toxin is antigenic and is neutralized by antibody found in convalescent sera. This property has been used in the diagnosis of scarlet fever (Dick test and Schultz Charlton reaction) which is no longer a common disease.
7. Haemolysin:
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Beta haemolytic streptococci of group A elaborate two haemolysins (Streptolysins). Streptolysin O is a protein which combines with anti-streptolysin O, an antibody that appears in human following infection with any streptococci. This antibody blocks haemolysis by streptolysin O and form a basis of a quantitative test for the antibody.
An anti-streptolysin O (ASO) serum titre in excess of 160-200 units is considered high and suggests recent infection.
Streptolysin S is responsible for haemolysis on blood agar medium and is not antigenic.
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(1) Erysipelas:
When haemolytic group A streptococci enter the skin, erysipelas results with massive brawny oedema and a rapidly advancing margin of infection.
(2) Puerperal Fever:
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If they enter the uterus after delivery, puerperal fever develops, which is essentially septicaemia.
(3) Sepsis:
Infection of surgical wound results in surgical scarlet fever.
(4) Sore Throat:
When they adhere to the pharyngeal epithelium by means of their pili, the sore throat occurs.
(5) Pyoderma:
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Local infection of superficial layers of skin in children is called impetigo. It consists of superficial blisters that break down and erode areas whose denuded surface is covered with pus and or crust.
8. Infective Endocarditis:
In course of acute bacteriaemia, beta haemolytic streptococci and pneumococci may settle on normal or previously deformed heart valve and produce acute endocarditis. Rapid destruction of the valve leads to fatal cardiac failure.
9. Sub-Acute Endocarditis:
It involves abnormal valves (congenital deformities and rheumatic lesions).
10. Post-Streptococcal Disease:
(Rheumatic fever, glomerulonephritis) Following an acute infection, there is a latent period of 1-4 weeks after which nephritis or rheumatic fever occasionally develops. Nephritis is more commonly preceded by the infection of the skin; rheumatic fever, by infection of the respiratory tract.
(1) Acute Glomerulonephritis:
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It may be initiated by antigen antibody complexes on the glomerular basement membrane. In acute nephritis, there is blood, protein in the urine, oedema, high blood pressure and urea nitrogen retention; serum complements are low. Some patients develop chronic glomerulonephritis with ultimate kidney failure.
(2) Rheumatic Fever:
Certain strains of group A streptococci contain cell membrane antigens that react with human heart tissue antigens, thereby heart muscle and valves are damaged. Typical symptoms and signs of rheumatic fever include fever, malaise, polyarthritis and carditis.
11. Laboratory Diagnosis of Streptococci:
A. Specimens:
Throat swab, pus or blood is obtained for culture and serum for serology.
B. Smears:
Smears from pus show Gram-positive cocci in chains; cocci which are no longer viable are sometimes Gram-negative.
C. Culture:
They are grown on Blood agar medium. Incubation in 10% CO2 often speeds haemolysis. Blood cultures in cases of suspected endocarditis may turn positive after a week or longer as alpha haemolytic streptococci and enterococci may grow slowly.
12. Serological Tests of Streptococci:
Several commercial kits are available for rapid detection of group A streptococci antigen from throat swab. ELISA and agglutination tests can demonstrate the presence of antigen. These tests can be completed in 1-4 hours after the specimen is obtained. They are more sensitive and specific when compared to culture test.
In respiratory disease, a rise in titre of antibodies can be estimated by ASO title which is most widely used. In skin infection, anti-hyaluronidase test is useful. The streptozyme test is passive slide haemagglutination test using erythrocytes sensitized with a crude preparation of extracellular antigen of streptococci. It is a sensitive specific screening test for skin or throat infection.
13. Treatment of Streptococci:
They are sensitive to penicillin, erythromycin. Antimicrobial drugs have no effect on glomerulonephritis and rheumatic fever.
