FIG. 5.

F1 antigen-specific IgG responses induced in individual mice following immunization with Salmonella expressing F1 antigen. Groups of BALB/c mice (n = 6) were orally immunized with a single dose of 1 × 10¹⁰ to 5 × 10¹⁰ CFU of ORT strain SLDAPD/pAHL or original vaccine strain SL3261/pAH34L. Serum samples were removed 41 days after inoculation, and IgG responses were measured by ELISA. The results are expressed as the reciprocal of the end point dilution for each animal group. The limit of detection is indicated by the horizontal dashed line. Mice were subsequently challenged with 185 MLD (A) or 1.85 × 10⁵ MLD (B) of Y. pestis GB. The asterisks indicate mice that did not survive the Y. pestis infection.