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Microbiology

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Gram Positive Cocci

Question
30 out of 78
 

Transport media for Streptococcus pyogenes is:



A Pike'smedium
B Blood agar

C Ludlams medium
D Chocolate agar

Ans. A Pike's medium

a. The transport medium used for group A streptococcus is Pike’s medium.

b. Crystal violet blood agar similar in constituents to pike’s is used as the selective medium.

c. Chocolate agar and blood agar are enriched media.

d. Ludlam’s medium is a selective medium for S.aureus.

EXTRA EDGE

ON VIRULENCE FACTORS OF S. PYOGENES

Virulence factor

Activity

Lipoteichoic acid

Mediates adherence to epithelial cells

M protein

Mediates adherence to epithelial cells, inhibits phagocytosis

Hyaluronic acid capsule

Inhibits phagocytosis

Erythrogenic toxin (Streptococcal pyrogenic exotoxin)

There are three distinct toxins A, B, C (SPE A, B and C). Type A is encoded by bacteriophage gene speA, type B toxin by chromosomal gene speB and toxin C by bacteriophage gene speC. These toxins are superantigens , T cell mitogens that induce lymphocytes to synthesize and release cytokines which are responsible for fever shock and tissue damage

Streptolysin O

In addition to group A streptococci, it is also produced by some group C and G streptococci. It is a heat labile protein. It acts by binding to cholesterol in the cell membrane and producing holes in it. It is inactivated by oxygen. It is strongly antigenic and demonstration of antibodies (Antistreptolysin O) against it indicates recent group A streptococcal infection.

Streptolysin S

This hemolysin is not inactivated by oxygen and is responsible for hemolysis on the surface of an aerobic blood agar plate. Due to its small size it is not antigenic. It also has leucocidal action.

Streptokinase

Produced by group A, C and G streptococci and is antigenic. It has fibrinolytic action and is thought to be responsible for rapid spread of infection by preventing the formation of fibrin barrier. This property is also used therapeutically in treatment of coronary thrombosis.

Deoxyribonuclease

Four different types have been identified (A,B,C and D). They hydrolyze nucleic acids and nucleoproteins. They are antigenic and DNAase B is the most common form produced by S. pyogenes. Anti-DNAase B is useful for the retrospective diagnosis of skin infections, where antistreptolysin titre may be low.

Hyaluronidase

Produced by strains of group A, B, C and G streptococci. It is antigenic. It is thought to play an important role in spread of infection through the tissues.

Serum opacity factor

It is a lipoproteinase enzyme. It reacts with and produces opacity in mammalian sera. It is produced by group A streptococci of certain M types. It is loosely bound to the cell and is antigenic.

Gram Positive Cocci Flashcard List

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