Figure 8. Propionate does not impair passive immunization and protection to secondary infection.

Anti-K. pneumoniae (A), anti-S. pneumoniae (B) and anti-C. albicans (C) IgG titers in BALB/c mice surviving infection with 20 CFU K. pneumoniae (n = 4 control and 5 propionate-treated mice; Fig. 5B), 10⁴ CFU S. pneumoniae (n = 9 control and 10 propionate-treated mice; Fig. 7C) and 4 × 10⁴ CFU C. albicans (n = 9 control and 9 propionate-treated mice, serum was collected 3 weeks post-infection). Box and min-to-max whisker plots represent the OD_{450 nm} using plasma (diluted 1/200) collected on day 21 after infection. P = 0.1, 0.01 and 0.02, respectively. No signal was detected using plasma from uninfected mice. (D,E) BALB/c mice (n = 18–21 per group) were treated with or without 200 mM propionate in drinking water for 3 weeks, challenged i.n. with 80 CFU S. pneumoniae, and used for subsequent experimentation 3 weeks later. (D) Sera collected from 8 water and 11 propionate-treated mice were pooled and transferred (120 μl i.p.) into naive mice (n = 10 per group) infected 24 h later with 4 × 10⁶ CFU S. pneumoniae (~100 x LD₁₀₀). Survival was monitored for 21 days. P = 0.6. (E) Propionate treatment was withdrawn. Mice (n = 10 per group) were infected with 10⁷ CFU S. pneumoniae (~250 x LD₁₀₀). Survival was monitored for 21 days. P = 0.8.