Enterobactin-based Immune Interventions against Gram-negative Bacterial Infections

Enterobactin (Ent)-mediated iron acquisition is critical for many Gram-negative bacteria to survive in the host. Given the bacteriostatic effect of lipocalin-2 resulting from its potent Ent-binding ability, immune interventions directly targeting Ent is a promising antimicrobial strategy against Gram-negative bacterial infections. Moreover, hyperimmune egg yolk antibody is an emerging passive immune agent for the control of bacterial infections. Oral administration of anti-Ent egg yolk antibody may confer passive immune protection against Ent-dependent enteric pathogens. In this dissertation study, multidisciplinary approaches in conjunction with different model systems (in vitro, in vivo, and ex vivo) were used to examine protective efficacies of the Ent-based immune interventions. Anti-Ent antibodies induced by an Ent conjugate vaccine were demonstrated to exert in vitro bacteriostatic effect on Ent-dependent Gram-negative bacterial pathogens, including diverse Escherichia coli and Salmonella enterica strains with different siderophore production profiles as well as Campylobacter spp. In the subsequent chicken trial aimed to assess active immune protection of the Ent conjugate vaccine against avian pathogenic E. coli (APEC) infection, the vaccine induced high titers of anti-Ent IgY in the serum as well as specific IgY in the intestine. As a result, the vaccination significantly alleviated the organ lesions caused by a highly virulent APEC and affected the chicken intestinal microbiota. Regarding the passive immunization, when a large quantity of anti-Ent hyperimmune egg yolk was orally administered as a feed additive to chickens, it failed to suppress C. jejuni colonization in the chicken intestine, consistent with the finding that the egg yolk antibody was not detected in gastrointestinal tracts and sera of the chickens. Further ex vivo evaluation of egg yolk antibody stability revealed that the antibody was efficiently degraded when entering chicken gizzard. Lastly, an epidemiological survey of E. albertii in broilers revealed chickens as an important reservoir for this emerging zoonotic pathogen. The in vitro growth assay also showed significant inhibitory effect of the anti-Ent antibodies on E. albertii. Collectively, this dissertation study demonstrated significant potential of the Ent-based immune interventions in controlling different Gram-negative bacterial infections.