Characterization and Immunoprotective Efficacy of a Fumarate reductase FrdA Mutant of Salmonella Enteritidis

The TCA cycle can participate in several physiological processes of Salmonella and is closely associated with bacterial pathogenicity; however, it remains unclear whether frdA, a key enzyme of the TCA cycle, is involved in the Salmonella infection process. To determine the role of fumarate reductase frdA in Salmonella infection, a frdA-gene deletion strain of Salmonella enteritidis (S. enteritidis) was generated in this study, and the effect of frdA knockout on the biological properties and pathogenicity of S. enteritidis were further examined. The results showed that frdA deletion did not affect the growth and biochemical properties of S. enteritidis but caused a significant decrease in the survival ability under environmental stress, as well as a substantial decrease in its motility and biofilm formation ability. The animal infection test showed that the frdA gene deletion could lead to a significant decrease in virulence of Salmonella enteritidis in mice; the LD50 for mice increased nearly 100-fold compared with that of the parental strain, with downregulated expression of csgD and other virulence genes. Moreover, oral immunization of mice with the frdA deletion strain could provide promising immune protection for mice. Thus, frdA is closely associated with the virulence of Salmonella enterica and is an attractive candidate target for vaccine design of Salmonella.