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Microbiology

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Gram Positive Cocci

Question
24 out of 78
 

A woman had a high fever, hypotension, and a diffuse macular rash. When all cultures showed no bacterial growth, a diagnosis of toxic shock syndrome was made. Regarding the mechanism by which the toxin causes this disease, which one of the following is LEAST accurate? (AIPG 2011)



A The toxin is not processed within the macrophage.

B The toxin binds to both the class II MHC protein and the T cell receptor.

C The toxin activates many CD4-positive T cells, and large amounts of interleukins are releaseD.

D The toxin has an A-B subunit structure--the B subunit binds to a receptor, and the A subunit enters the cells and activates them.

Ans. D The toxin has an A-B subunit structure--the B subunit binds to a receptor, and the A subunit enters the cells and activates them.

Pyrogenic toxin super antigens are powerful immunostimulatory exotoxins produced by S. aureus and S. pyogenes

a. TSST-1

b. STAPHYLOCOCCAL ENTEROTOXINS-SEA, B,Cn,D,E, G,H

c. STREPTOCOCCAL PYROGENIC EXOTOXINS-SPE A, B, C

i. A unique feature of superantigens is that, unlike conventional antigens, they do not require processing by the Antigen Presenting Cells and they can interact with a large number of T cells that share particular sequences with Vb elements of the T Cell Receptor.

ii. Therefore, by comparison to antigens that may interact with 1 cell in every 104 or 106 T cells, superantigens are capable of interacting with as many as 5 to 20% of resting T cells with no MHC restriction.

iii. The binding of superantigens to the TCR triggers intracellular biochemical signals that program a number of events leading to cell activation, differentiation, proliferation, and the release of inflammatory cytokines

STREPTOCOCCUS

Extra Edge

I. Discovered by Billroth (1874)

II. Gram positive cocci in chains. Nonmotile and nonsporing.

III. Facultative anaerobes (some are obligate anaerobes).

IV. Catalase and oxidase negative.

V. Grow poorly on basal media, growth enhanced on enriched media (addition of glucose, blood or serum)

VI. Form part of normal flora of man and animals

VII. Most important species is S. pyogenes (group A beta hemolytic streptococci).

Gram Positive Cocci Flashcard List

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