• The genus streptococcus include a large number of species of Gram +ve, catalase –ve cocci arranged in chain of varying length and also in pairs.
• Streptococcus form chains during growth due to successive cell division occurring in one plane only and daughter cells failing to separate completely.

Habitat:

• Some species are part of normal flora of Human and animals. Streptococcus salivarius
• some are important human pathogens eg. Streptococcus pyogens, S. pneumoniae, S agalactiae.

Morphology:

• The individual cocci are spherical or ovoid, 0.5-1 µm in diameter and are arranged in chains.
• The length of chain vary widely with cultural conditions. Larger chain are formed in liquid or broth than in solid agar.
• Streptococci are gram positive but they may become gram negative in ageing when bacteria die.
• Streptococci are non-motile and non-spore forming.
• Most species possess capsule. Group A and group C streptococci possess hyaluronic acid capsule while group B and D possess polysaccharide capsule.

Cultural and biochemical characteristics:

• Streptococci are aerobic or facultative anaerobes.
• They grow best at temperature 37°
• Streptococci grow well in media containing blood and sugar. 10 % CO2 promotes growth and haemolysis.
• Selective media containing aminoglycosides or 1: 500,000 crystal violet selectively permit growth of Streptococci by inhibiting other bacteria.
• On blood Agar: colonies are small, circular, semi-transparent, convex with small clear zone of hemolysis. Growth and hemolysis increases y 10% CO2. Streptococci are classified on the basis of hemolysis, α-hemolysis, β-hemolysis and no-hemolysis group.
• Streptococci are non-halophilic but can tolerate up to 6.5% NaCl (halotolerant)
• Tolerates 0.1% methylene blue and bile esculin agar.
• Streptococci ferment most sugars such as sorbitol. Trehalose, lactose maltose,mannitol etc with production of acid but no gas.
• Catalase (–­) negative

Genus Streptococcus: habitat, morphology, culture and biochemical characteristics

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Virulence factors of streptococcus:

1. Capsular hyaluronic acid
2. Group specific polysaccharide antigen
3. Type specific antigen
4. Protein F and G
5. Exotoxins
6. Hemolysin: Streptolysin
7. Enzymes

1. Capsular Hyaluronic acid:

• Hyaluronic acid capsule are present in group A and group C streptococci.
• The capsule is not immunogenic
• It protects the bacteria against phagocytosis.

2. Group specific polysaccharide antigen:

• Group specific polysaccharide antigen plays a pathogenic role in streptococcus infection.
• These antigens are chemically similar in structure and composition to human tissues particularly to those of heart, kidney and joints.
• As a result the immune response toward streptococci become auto-reactive and causes endocarditis, glomerulonephritis or arthritis.

3. Type specific antigens:

i. M-protein:

• It is associated with fimbriae on cell wall.
• It is heat and acid resistant but susceptible to tryptic digestion and soluble in alcohol.
• M-protein promotes adherence and enhance virulence.
• It also protect phagocytosis.
• Streptococci without M-protein is non-virulent.

ii. T and R protein:

• T and R proteins have no relation to virulence
• They are heat labile protein.
• Some pathogenic S. pyogens possess R protein

4. Protein F and G:

• Protein –F promotes binding to pharyngeal epithelial cell. It is fibronectin binding protein.
• Protein-G bind to Fc receptor of IgG and prevents antibody mediated phagocytosis.

5. Exotoxins:

i. Erythrogenic toxins:

• It is immunogenic.
• It causes characteristic rashes and scarlet fever.

ii. Exotoxin A:

• It is associated with toxic shock syndrome (TSS) caused by S.aureus.
• It is a super antigen.

iii. Exo-toxin B:

• It is a cysteine protease and responsible for tissue destruction.

iv. Cardio-hepatic toxin:

• It is responsible for heart and liver dysfunction.

6. Hemolysin: Streptoysin

• Most group A and some group C and G streptococci produces hemolysin.
• It is also known as streptolysin.

a) Streptolysin O:

• It is an Oxygen labile hemolysin.
• Streptolysin O lyses RBCs by binding to cholesterol containing cell membrane/
• It causes complete hemolysis around colony in Blood agar
• Weight 60,000 Da
• Anti-streptolysin O appears on serum of human following streptococcus infection.

b) Streptolysin S:

• It is an oxygen stable hemolysin.
• It causes surface colony hemolysis
• It is lyses RBCs and it has also leucocidal action.
• It is non-antigenic.

7. Enzymes:

i. Streptokinase (fibrinolysin):

• Streptokinase is a fibrinolysin and promotes lysis of blood clot.
• Two types; streptokinase A and B.
• Streptokinase promotes spread of streptococci infection by preventing formation of clot barriers.

ii. Streptococcal deoxyribonuclease (DNase):

• Streptococcal DNase degrade host cell DNA.
• There are 4-types of DNase enzymes-type A, B, C and D.
• DNase liquefies viscus DNA in pus.
• Nicotinamide adenine denucleotidase (NADase):
• NADase acts as co-enzyme.
• It is antigenic and has leuco-toxic effects.

iii. Hyaluronidase: It is antigenic

iv. Lipoproteinase: It is antigenic

v. Neuraminidase

vi. Lipase

vii. Phosphatase

viii. Esterase

ix. NAGase

x. Amylase

Streptococcal infections

1. Sore throat: acute tonsillitis and pharyngitis
2. Scarlet fever: it occurs as a complication of sore throat. And characterized by generalized erythematous rashes
3. Impetigo: superficial discrete crushed spot especially in children. It lasts for 1-2 weeks and heal spontaneously without scar.
4. Erysipelas: it is an acute spreading lesion of skin. Infected areas of skin shows massive edema with erythema.
5. Cellulitis: it is sub-cutaneous infection
6. Sepsis
7. Otitis media
8. Bacteremia
9. Pneumonia
10. Streptococcus shock syndrome
11. Rheumatic fever

Virulence factors and streptococcal infections

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1. Classification on the basis of Oxygen requirement
2. Brown classification; on the basis of haemolytic pattern on sheep blood agar
3. Shermann’s classification; on the basis of physiological characteristics
4. Lancifield classification; serological classification
5. Biochemical classification
6. Classification on the basis of 16s rRNA sequence

1. Classification on the basis of Oxygen requirement:

i. Aerobic or facultative anaerobes: eg. Streptococcus spp

ii. Obligate anaerobes: eg. Peptostreptococcus

2. Brown classification: On the basis of haemolytic pattern on sheep blood agar.

i. Αlpha-haemolysis group:

• Form incomplete haemolysis on blood agar
• Shows greenish discoloration around colony and persistence of some unhaemolyszed RBCs.
• Streptococcus pneumoniae, Viridians streptococci

ii. Βeta-haemolysis group:

• Form complete haemolysis on blood agar
• Give 2-3 mm diameter zome of haemolysis
• Streptococcus pyogens

iii. Non-haemolysis group

• Does not cause haemolysis at all
• These are non-haemolytic group
• Streptococcus faecalis

3. Shermann’s classification: on the basis of physiological characteristics

i. Pyogenic streptococci: Streptococcus pyogens

ii. Lactococci

• Found in dairy products
• They are non-haemolytic group
• eg. Lactoccus

iii. Enterococci:

• They are normal flora of human intestine
• Enterococcus

iv. Viridans streptococci

• They are normal flora of upper respiratory tract of human

 4. Lancefield classification: serological classification

• This classification is based on the difference in the structure of cell wall carbohydrate ie. group specific polysaccharide antigen. Most strain of β-haemolytic group and some strain of α- hemolytic and non-haemolytic group are classified on the basis of cell wall polysaccharide.
• Streptococci are classified into 20 lancifield group from A to V except I and J.

Group A; Streptococcus pyogens

Group B; Streptococcus agalactiae

Group C; Streptoccus equi

Group D; Enterococcus

other (group E to V)

• Group A streptococci ie. S.pyogens is further sub divided into approximately 80 serotypes by Griffith according to their specific surface protein (M, T and R). M-protein is the most important one.
• The pathogenicity of Streptococci depends on the presence of hyaluronic acid capsule and surface M-protein.

5. Biochemical classification:

i. On the basis of sugar fermentation test;

• Streptococci ferments most sugar with production of lactic acid but no gas.
• Accumulation of lactic acid in media terminates bacterial growth.
• Streptococcus pyogens do not ferment ribose and they give PYR test positive.

ii. On the basis of production of certain enzymes

iii. On the basis of antibiotic susceptibility test

iv. On the basis of colony characteristics on different media

v. On the basis of haemolysis pattern

6.  Classification on the basis of analysis of 16s rRNA sequence

i. Pyogenic group: eg. Streptococcus pyogens, S. agalactiae

ii. Anginosius group; eg. Streptococcus anginosus, S. intermedius

iii. Mitis group: eg. Streptococcus mitis, S. pneumoniae

iv. Salivarious group: eg. Streptococcus salivarious, S. thermophiles

v. Bovis group: eg. Streptococcus bovis, S. equines

vi. Mutans group: eg. Streptococcus mutans, S. ferus

Classification of Streptococcus

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