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S. pyogenes is "flesh-eating" bacteria. It results from life-theatening myonecrosis caused by this organism. S. pyogenes avoids phagocytosis (mediated primarily by capsule, M and M-like proteins, C5a peptidase), adhere to and invade host cells (M protein, lipoteichoic, F protein), and produce toxins (streptococcal pyrogenic exotoxins, steptolysin S, streptolsin O, streptokinase, DNases).

Suppurative diseases: pharyngitis, soft-tissue infections.

Erysipelas: localised skin infection with pain, inflammation, lymph node enlargement and systemic symptoms.

Cellulitis: infection of the skin that involves the subcutaneous tissues.

Necrotising fasciitis: deep infection of skin that involves destruction of muscle and fat layers.

I think these are most its skin infections. But what about its diagnostics?

I think take biopsy from living tissue, not from necrotic. If pus, then probably from it. I think we can do Gram staining, recognise gram-positive cocci like in a string Then, we can do hydrogen peroxide test - if no bubbles, then suspicion to S. pyogenes, since catalase negative. The do optochin test which is positive for S. pyogenes.

What is the correct procedure for doing skin infection diagnostics for group A streptococci?

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S. pyogenes cultured on Blood Agar should show beta-hemolysis. B-hemolytic colonies can be further identified as S. pyogenes by the results of negative catalase (hydrogen peroxide) test, positive L-pyrrolidonyl arylamidase (PYR) reaction (1) and sensitivity to Bacitracin (2). See https://www.inkling.com/read/medical-microbiology-murray-rosenthal-pfaller-7th/chapter-19/streptococcus-pyogenes

PCR may also be used: http://www.sciencedirect.com/science/article/pii/S0923250805000215

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