Growth of typhoid and paratyphoid bacilli in intravenously infected mice.

The in vivo growth of Salmonella paratyphi A, S. paratyphi B, S. paratyphi C, and S. typhi, as well as of an S. typhi-typhimurium hybrid, was studied in three different strains of mice. S. paratyphi A and B and S. typhi demonstrated very little growth potential in any of the intravenously infected mice, even after as many as 20 serial mouse passages. It was noted, however, that small numbers of viable S. paratyphi B and S. typhi persisted in the spleens of infected mice for up to 28 days. Salmonella paratyphi C and the S. typhi-typhimurium hybrid gave rise to progressive systemic infections beginning from very small intravenous inocula. The median lethal doses for the C57B1 strain of mouse were about five organisms. The relevance of these findings with regard to the development of an animal model for studying human typhoid fever vaccines is discussed.