Development and efficacy of a novobiocin-resistant Streptococcus iniae as a novel vaccine in Nile tilapia (Oreochromis niloticus)

A novel attenuated Streptococcus iniae vaccine was developed from a virulent strain of Streptococcus iniae (ISET0901) through selection for novobiocin resistance (named ISNO). The safety of ISNO was then evaluated in Nile tilapia (Oreochromis niloticus) through intraperitoneal (IP) injection. When male tilapia (average weight 10 g) were IP injected with 2 × 10⁷ colony-forming units (CFU) of the attenuated S. iniae vaccine strain, no fish died. However, when the same age and size matched tilapia were IP injected with 2 × 10⁷ and 1 × 10⁵ CFU of the virulent parent strain of S. iniae, 100 and 90% fish died, respectively. Backpassage safety studies revealed that ISNO was unable to revert back to a virulent state. When IP vaccinated fish were challenged by the virulent ISET0901 strain of S. iniae, relative percent survival (RPS) values of vaccinated fish at 14, 28, 60, 90, and 180 days post ISNO vaccination (dpv) were 100, 100, 100, 89, and 75%, respectively, The RPS values of ISNO vaccinated fish (IP vaccination) against infections by five heterologous virulent strains of S. iniae (F3CB, 102F1K, 405F1K, IF6, and ARS60) at 60 dpv were 78, 90, 100, 100, and 100%, respectively. When tilapia were IP vaccinated by ISNO at dose of 1 × 10², 1 × 10³, 1 × 10⁴, 1 × 10⁵, 1 × 10⁶, and 1 × 10⁷ CFU/fish, RPS values at 28 dpv were 81, 94, 100, 100, 100, and 100%, respectively. At 28 dpv, RPS of vaccinated fish by ISNO through bath immersion (1 × 10⁷ CFU/ml) was 88%. ELISA results revealed that protection elicited by ISNO was due to antibody- as well as cell- mediated immunity. Our results suggest that ISNO could be used as a novel safe and efficacious vaccine to protect Nile tilapia from S. iniae infections.