FIG 4.

Oral immunization of hamsters with NTCD_mTcd138 spores induced protective responses. Groups of golden Syrian hamsters (n = 10) were orally immunized with NTCD_mTcd138 at 2 × 10⁶ spores 3 times at 14-day intervals. Sera and feces were collected after each immunization. Anti-TcdA/anti-TcdB IgG titers in sera and feces were determined by ELISA. (A, B) Immunization with NTCD_mTcd138 induced systemic toxin-specific antibody responses. (C, D) Immunization with NTCD_mTcd138 induced mucosal and systemic antibody responses against FliCD. (E, F) Vero cells were used to determine in vitro neutralizing activities of sera (E) or feces (F). The neutralizing titer is expressed as the maximum dilution of the sera that inhibits cell rounding caused by toxin at a given concentration. This given concentration is the minimum toxin dose causing cell rounding after a 16-hour of toxin exposure, i.e., 2.5 and 0.1 ng/ml for TcdA and TcdB, respectively. Bars stand for means ± SD. *, P < 0.05 versus 1st IM in panels A, B, C, and D; ns, no significance versus anti-TcdA, P > 0.05 in panels E and F. Student's unpaired t test was used for statistical significance.