SUMMARY: The LD50 for mice of twenty-one strains of from human infection ranged from 2 to 600 million living bacilii. Mouse-virulence was not associated with the severity or the site of infection in the human subject.

The relatively high virulence of some strains was not due to any peculiar demonstrable toxicity of the cell-substance, because killed cells of high and low virulent strains were about equally toxic. The toxicity appeared to reside in a predominantly lipe-polysaceharide fraction, immunologically similar to the antigen characterizing the somatic surface of recently isolated strains.

This investigation was made to find whether strains of isolated from infections in man could be distinguished by a laboratory virulence test, and whether virulence could be correlated with any other microbiological character of the strains.


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