Virulence ofSalmonella typhimurium

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Abstract
A study of the virulence of S and R strains of S. typhi murium was made in an attempt to evaluate as possible determinants of virulence other biol. characters, viz., invasiveness, resistance to phagocytosis, ability to survive, and to multiply in the host. When Swiss mice from a uniform inbred stock were inoculated intraperitoneally with increasing doses, the increase in mortality was much more rapid within a certain range than the work of other investigators had indicated. The slope of dose-mortality curves seemed to depend more upon the degree of uniformity of the test animals with respect to susceptibility than upon the infective capacities of the test cultures. Virulence tests with mice of various degrees of susceptibility showed that large differences in virulence may not be apparent if highly susceptible mice are inoculated over only a narrow range of doses. The character of the dose-mortality curve and the susceptibility of the test animals must be known in order completely to describe the virulence of any particular strain. An S strain of high virulence and an S strain of low virulence, indistinguishable in cultural, serological, immunizing, and toxigenic characters, did not differ significantly in invasiveness and resistance to phagocytosis, but the ability of the virulent strain to multiply in the host was markedly greater. An avirulent R strain, although less invasive and less able to multiply in the host than S strains, survived better in the spleen. Ability to multiply in the host appears to be a major determinant of virulence.