Recently, vaccines based on outer membrane vesicles (OMVs) have attracted increasing attentions in the prevention and treatment of microbial infections. OMVs are immunogenic spherosomes which are secreted by Gram-negative bacteria carrying with the parent bacteria lipoprotein, lipopolysaccharide (LPS) and so on [
68]. These inherited substances could perform as pathogen associated molecular patterns (PAMPs) with the host cell surface expression pattern recognition receptors (PRRs, such as toll-like receptors (TLRs) and intracellular receptor response, to launch numerous immune responses [
69]. OMVs generated from various bacteria including
Pseudomonas aeruginosa [
70],
Salmonella typhimurium [
71],
Klebsiella pneumoniae [
72], and
Shigellae [
73] are thus widely considered as candidates for the new generation of vaccines due to their highly biocompatible nanostructures and naturally abundant immunogenic components [
74]. For instance, vaccines derived from
Neisseria meningitidis-OMVs have been licensed worldwide for the control of meningococcal B disease in human [
75]. Using
E. coli as the model pathogen, Gao and co-workers have successfully developed gold nanoparticles (BM-AuNPs) with an average diameter of about 30 nm which were coated with OMVs secreted from
E. coli (
Fig. 3) [
76]. After being subcutaneously injected, the as-prepared BM-AuNPs were able to move to neighboring drainage lymph nodes as well as induce rapid activation and maturation of dendritic cells in mouse lymph nodes. In addition, BM-AuNPs enhanced the secretion of interferon-gamma (INF-
γ) and interleukin-17 (IL-17). The results showed that BM-Au NPs could effectively induce the immune responses of bacteria-specific B and T cells in inoculated animals. Experimental studies showed that mice injected with BM-AuNPs coated with OMVs had significantly higher number of dendritic cells separated from lymphocytes than mice treated with PBS, suggesting that an increased concentration of dendritic cells in these lymph nodes is occurred. At the same time, the upregulation of co-stimulators CD40, CD80 and CD86 indicated that the isolated dendritic cells in the lymph nodes changed from immature to mature. Subsequently, the induction of specific antibodies to
E. coli was examined to evaluate the bacteria-specific B cell response. The obtained results showed that the mice treated with BM-AuNPs showed continuous elevation of corresponding antibodies, implying that the BM-AuNPs injection also significantly enhanced the specific B-cell immune response. Last, they also confirmed the T cell activation effect of BM-AuNPs to stimulate the body's immune response to bacteria, as the levels of T cell based immune cytokines including IL-17 and IFN-
γ were significantly increased. The results revealed that the as-prepared BM-AuNPs which were synthesized by a top-to-bottom method could preserve the immunogenic antigens with immunoadjuvant properties on the bacterial membranes to perform as bionic bacteria for triggering specific antibacterial immune responses.