FIG. 1.

E. muris infection generates dose-dependent immunity to IOE challenge infection. a. C57BL/6 mice were infected with 4 × 10⁴ E. muris, and 30 days later infected and uninfected control mice were challenged with high-dose IOE (2× LD₅₀). Morbidity was monitored using the criteria described in Materials and Methods. Four mice were used per group. b. Bacterial infection in the spleens of immunized and control mice was determined on the indicated days postinfection. Standard deviations of the means are indicated. c. Mice were immunized with the indicated doses of E. muris and challenged with high-dose IOE. d. IOE challenge infections of immunized mice were performed using from 1 to 8 times the LD₅₀, as indicated. In all experiments at least four mice were used in each group. e. Mice were infected via the peritoneum with L. monocytogenes (Lm; 7.2 × 10⁴ CFU) 7 days prior to challenge with IOE. Control mice received only Lm or IOE, or were immunized prior to IOE challenge with E. muris (Em-IOE). f. Mice were immunized via the peritoneum with E. muris and challenged with IOE via the peritoneum (i.p.) or intravenously (i.v.). The protection observed following either route of injection was statistically significant (P < 0.04).