Fig. 6.

Titres of anti-PA and anti-spore antibodies in guinea pigs before challenge with Ames spores. The animals were vaccinated three times at 4-week intervals with PA, PA+spore antigens (PA+Sp) or PBS alone (controls). Sera were collected from each animal 4 weeks after the first dose (a, c, e) and 4 weeks after the third dose (b, d, f, just before challenge with wild-type Ames spores at week 12). ELISAs were carried out on plates coated with PA (a, b), wild-type Ames spores (c, d) or ΔbclA spores (e, f). The results are the geometric mean titres (±sem) in µg PA ml⁻¹ and anti-spore antibody levels (mean A₄₀₅ and sd). The anti-PA titres (a, b) of the PA-only group were significantly higher than those of the PA+Sp group at week 4 (P = 0.0044), but the reverse was true for sera collected just prior to challenge at week 12 (P = 0.0054). Anti-spore antibody titres in sera collected at week 4 (c, e) and week 12 (d, f) were determined in ELISAs against wild-type (c, d) or ΔbclA (e, f) spores of Ames. The responses (c–f) of the PA+Sp vaccine group were significantly greater than those of the PA-only groups with mean reciprocal end-point dilution titres of up to 51 200 (P = 0.0053) for (c), ≤12 800 (P<0.033) for (d), through all reciprocal dilutions tested (P≤0.0007) for (e) and up to 51 200 (P<0.017) for (f), as determined by t tests with stepdown Bonferroni adjustment for multiple comparisons.