SUMMARY: Mice were challenged by mouth with a suspension containing equal numbers of streptomycin-sensitive (Str) and streptomycin-resistant (Str) variants of . These variants were of equal virulence but the Str variant grew more slowly than the Str variant. The LD50 dose contained 5 x 10 bacteria. Heart blood obtained from mice dying from many LD50 doses nearly always contained a great excess of the Str variant, but blood from mice dying from less than one LD50 dose contained either Str, Str, or a mixture of Str and Str variants. The appearance of the Str variant alone in the latter mice strongly suggests that these fatal infections were initiated by a very small number of organisms or possibly by a single organism. It is therefore concluded that these organisms were acting independently. In this system, it is likely that any bacterium which enters the tissues from the gut can initiate a fatal infection and that the probability of effecting such an entrance almost entirely determines the probability of an inoculated bacterium causing a fatal infection.


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