Virulence comparison of the WT and the arcA mutant in 6-8 week old C57BL/6 mice. (A) Single infection assays, where two groups of five mice per strain (WT and arcA mutant) were challenged intraperitoneally using 250 CFU/mouse, as described in Materials and Methods. Percent survival is the number of mice surviving relative to the number of mice challenged at zero time; (B) Competitive infection assays, where groups of three 6-week-old mice were infected orally (p. o.) or i. p. with a 1:1 mixture of S. Typhimurium 14028 s and its isogenic arcA mutant. After 4 or 6 days following i.p. or p.o. infection, respectively, mice were euthanized and mesenteric lymph nodes (MLN), liver, and spleen were collected for enumeration of the WT and the mutant. The competitive index (CI) was calculated as described in the Materials and Methods.