BM-AuNPs eliciting strong bacterium-specific antibody responses in vivo. (a) Anti-E. coli IgG titres at day 21 (n = 6). Black lines indicate geometric means. Naive mice were monitored as a negative control (open circles). (b) Time course of anti-E. coli IgG titres in naive mice (open circles) and mice immunized with 0.2 μg antigen/dose BM-AuNPs (red squares), 0.2 μg antigen/dose OMVs (blue squares), 0.02 μg antigen/dose BM-AuNPs (red triangles), and 0.02 μg antigen/dose OMVs (blue triangles), respectively (n = 6). (c) Quantified avidity index of the anti-sera from immunized mice (0.2 μg antigen/dose) binding to E. coli bacteria (n = 6). *p < 0.05, **p < 0.01.