A precise method of assaving viruIence of Sfaphylo~occmis a u r a s for mice by intraperitmml injection with mucin is described; the resuIts indicated that most bacteriophage types of S. aprreus possessed similar virulence, an LDgo consisting of millions of orga~lisms. The variability in results of similar tests by previo~is authors i s thought to reside in the use of techniques allowing excessive variability.The Smith strain of S. alcreus is shown to be exceptionally virulent, requiring only one colony-form in^ unit to kill a mouse. However, the survival time of mice after inoculation with the Smith strain i s longcr than that of other strains giving equivalent mortality. An esplanation is suggested for this latter phenomenon, and the manner in which the Smith strain causes death is considered. IntroductionThe purpose of the work described in this paper was to inject strains of Staphylococcus aureus into mice in an attempt to obtain a reproducibIe test of virulence. When the work was started, certain bacteriophage types, notably 80/81, seemed to be more frequentlv associated with outbreaks of infection in hospitals OVilliams 1959) and, since no tests comparing the virulence of different bacteriophage types had been reported, we thought it of importance to investigate this. A preliminary review of the Iiterature revealed a
We confirmed the widespread occurrence in human sera of antibodies able to protect mice from the lethal effect of the Smith strain of Staphylococcus aureus injected intraperitoneally together with hog gastric mucin. We were unable to demonstrate that the antibody was active against other naturally occurring strains of S. aureus nor did such strains absorb the protective antibody from serum.Repeated passage of two of the above strains of S. aureus in the peritoneal cavities of mice produced more virulent daughter strains. Human sera containing protective antibodies against the Smith strain also protected mice against these daughter strains, but the daughter strains also were unable to absorb the protective antibody from human sera. This suggests the existence in human sera of more than one kind of antibody protective against staphylococci.
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