Development and efficacy of novobiocin and rifampicin-resistant Aeromonas hydrophila as novel vaccines in channel catfish and Nile tilapia.

Three attenuated Aeromonas hydrophila vaccines were developed from the virulent 2009 West Alabama isolates through selection for resistance to both novobiocin and rifampicin. When channel catfish (Ictalurus punctatus) were IP injected with 4×105 colony-forming unit (CFU) of the mutants, no fish died. However, when the same age and size matched channel catfish were IP injected with similar amount of their virulent parents, 80-100% fish died. Similarly, when Nile tilapia (Oreochromis niloticus) were IP injected with 2×108 CFU of the mutants, no fish died. However, when Nile tilapia were IP injected with similar amount of the mutants, all fish died. Vaccination of channel catfish with the mutants at dose of 4×105 CFU/fish offered 86-100% protection against their virulent parents at 14 days post vaccination (dpv). Vaccination of Nile tilapia with the mutants at dose of 2×108 CFU/fish offered 100% protection against their virulent parents at 14, 28, and 56 dpv. Agglutination assay results suggested that protection elicited by the mutants was partially due to antibody-mediated immunity. Taken together, our results suggest that the three attenuated vaccines might be used to protect channel catfish and Nile tilapia against the highly virulent 2009 West Alabama isolates of A. hydrophila.