Phenotypic characterization, virulence, and immunogenicity of Edwardsiella tarda LSE40 aroA mutant

Abstract

Bacterial aro mutants are frequently used as live attenuated vaccines for domestic animals. In this study, we characterized Edwardsiella tarda strain LSE40 with a deletion in the aroA gene. In addition to autotrophy, the aroA mutant appeared to have delayed cell division and reductions in its swarming motility, biofilm formation, and production of translocator proteins in the type III secretory system. The mutant exhibited high virulence attenuation in turbot fish, Scophthalmus maximus (L.), where the 50 % lethal dose increased by more than 3 log₁₀ via intraperitoneal (i.p.) injection and by >2 log₁₀ via immersion exposure compared with the wild-type parent strain. A tissue persistence study showed that the mutant retained the ability to invade and spread in turbot and viable cells could be detected up to 28 days after i.p. infection and 21 days after immersion exposure. These results suggested a pleiotropic role for aroA in the physiological behavior of E. tarda. Turbot exhibited a good humoral response and the enhanced expression of innate immune factors, interleukin 1β and lysozyme, when vaccinated with aroA mutant at 10⁵ CFU via i.p. injection and at 10⁸ CFU via immersion exposure. However, the aroA mutant did not provide effective protection for turbot against edwardsiellosis following i.p. vaccination at doses of 10⁴-10⁶ CFU or immersion vaccination at doses of 10⁶-10⁸ CFU ml⁻¹. We hypothesized that the aroA mutant did not trigger an appropriate T cell-immune response in turbot against infection of E. tarda.