Evaluation of immunogenicity and efficacy of the enterobactin conjugate vaccine in protecting chickens from colibacillosis

Colibacillosis is one of the most common and economically devastating infectious diseases in poultry production worldwide. Innovative universal vaccines are urgently needed to protect chickens from the infections caused by genetically diverse avian pathogenic Escherichia coli (APEC). Enterobactin (Ent) is a highly conserved siderophore required for E. coli iron acquisition and pathogenesis. The Ent-specific antibodies induced by a novel Ent conjugate vaccine significantly inhibited the in vitro growth of diverse APEC strains. In this study, White Leghorn chickens were immunized with the Ent conjugate vaccine using a crossed design with two variables, vaccination (with or without) and APEC challenge (O1, O78, or PBS control), resulting in six study groups (9 to 10 birds/group). The chickens were subcutaneously injected with the vaccine (100 μg per bird) at 7 days of age, followed by booster immunization at 21 days of age. The chickens were intratracheally challenged with an APEC strain (10⁸ CFU/bird) or PBS at 28 days of age. At 5 days post infection, all chickens were euthanized to examine lesions and APEC colonization of the major organs. Immunization of chickens with the Ent vaccine elicited a strong immune response with a 64-fold increase in the level of Ent-specific IgY in serum. The hypervirulent strain O78 caused extensive lesions in lung, air sac, heart, liver, and spleen with significantly reduced lesion scores observed in the vaccinated chickens. Interestingly, the vaccination did not significantly reduce APEC levels in the examined organs. The APEC O1 with low virulence only caused sporadic lesions in the organs in both vaccination and control groups. The Ent conjugate vaccine altered the bacterial community of the ileum and cecum. Taken together, the findings from this study showed the Ent conjugate vaccine could trigger a strong specific immune response and was promising to confer protection against APEC infection.