Summary: Filtrates from strains of possessing plasmid-cloned haemolysin (Hly) genes and from strains possessing ‘wild’ Hly plasmids were lethal for mice on intravenous inoculation; similar doses of preparations from derivatives of these strains in which the Hly genes had been rendered non-functional or which did not possess the ‘wild’ plasmids were not. Live cultures of both kinds of Hlystrain usually had a lower lethal dose for mice on intraperitoneal inoculation than the corresponding Hlyforms. Mice that had been inoculated with Hlyforms had shorter survival times and lower numbers of organisms in peritoneal washings, lungs and blood at point of death than mice that had been inoculated with the corresponding Hlyforms; this was also so for mice pre-treated with FeSO, a procedure which rendered mice equally susceptible to the lethal effects of the Hlyand Hlyforms of a strain. In FeSO-treated mice the numbers of organisms in the tissues of those dying from infection with Hlyorganisms were no higher than they were at the same time after inoculation in others given the corresponding Hlyforms; before mice of the latter category died the numbers of organisms in their tissues increased greatly. The clinical and pathological signs exhibited by mice inoculated with Hlyorganisms, but not with Hlyorganisms, resembled those exhibited by mice inoculated with bacteria-free haemolysin preparations. These results suggest that haemolysin played a significant role in the pathogenesis of the disease produced by the Hlyorganisms by having a direct toxic action on the host.


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