Relevance of inoculation route to virulence of three Salmonella spp. strains in mice.

The virulence of three Salmonella species strains was compared by the i.p. and i.v. routes in BALB/c mice. Salmonella choleraesuis, SL2824 (serogroup C1, O-6,7), was more virulent by the i.v. than i.p. route. A strain of S. typhimurium, SL1260 (serogroup B; O-1,4,12) was more virulent i.p. than i.v. while another strain, SL3201 (O-4,5,12) was equally virulent i.p. or i.v. The LD50 of each strain by either route correlated with the number of bacteria in the liver and spleen on day one after inoculation and thus seems determined mainly by initial bactericidal mechanisms. The rate of bacterial growth in the liver and spleen was independent of inoculation route but differed between the three strains. Salmonella choleraesuis multiplied faster than either strain of S. typhimurium. Non-virulent aromatic-dependent (aro) derivatives of these strains were tested, instead of their virulent ancestors, for survival within peritoneal macrophages in vitro. Salmonella choleraesuis SL 2824 aro and S. typhimurium SL1260 aro were much more readily killed intracellularly than S. typhimurium SL3201 aro. The data indicate that the survival and multiplication of different Salmonella serotypes or strains in vivo may depend on different critical properties or mechanisms to overcome host defenses.