The key to the puzzling species specificity of group A streptococci (GAS) is its production of streptokinase, which only activates the blood clot–dissolving factor plasminogen in humans, according to research published in Science this week.

The results suggest that this group of organisms, which includes the so-called flesh-eating bacterium—a highly invasive, rapidly progressing, and potentially fatal member of the GAS family that attacks soft tissues and internal organs—evades host attempts at containing infection by breaking down localized defensive blood clots and spreading through the blood stream and into surrounding tissues, according to Hongmin Sun, principal author of the study.

Bacterial streptokinase has previously been shown to interact only with the human version of plasminogen to activate its clot-dissolving function. It does not interact with plasminogen from mice or other mammals, suggesting a mechanism for specificity that the team tested by creating what they called "humanized mice" transgenic for the...

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