Development and efficacy of a novel streptomycin-resistant Flavobacterium johnsoniae vaccine in grass carp ( Ctenopharyngodon idella )

A novel attenuated Flavobacterium johnsoniae M170 vaccine was developed from a pathogenic F. johnsoniae M168 isolate through a streptomycin-resistant strategy. The safety of the attenuated F. johnsoniae M170 strain was evaluated in grass carp (Ctenopharyngodon idella) through intraperitoneal (IP) injection and bath immersion. When grass carp (mean weight 10g) were challenged by IP injection with 1×108 colony-forming units (CFU) per fish, or by immersion with 1×107CFU/ml of the attenuated F. johnsoniae M170, no mortality, adverse behavior, or signs of disease were observed. However, when grass carp were challenged by IP injection of 1×107CFU per fish, or by immersion with 1×107CFU/ml of F. johnsoniae virulent strain M168, the mortality of challenged fish were 100% and 90%. Back-passage safety studies indicated that the M170 strain cannot revert back to a pathogenic state after 5 passages. The values of relative percent survival (RPS) in the IP injection group were 100% and 60% when challenged at 28 and 240days post vaccination (dpv), while RPS in the bath immersion group were 73.1% and 34.8% when challenged at 28 and 240dpv. ELISA (enzyme-linked immuno sorbent assay) results revealed that F. johnsoniae M170 could induce an antibody mediated immunity response. The above results suggest that the attenuated F. johnsoniae vaccine M170 could be used to protect grass carp from F. johnsoniae infections.