The primers are listed in Table 1. Real-time cycling conditions were as follows: 95 °C for 30 s; 40 cycles of 95 °C for 5 s, 55 °C for 30 s and 72 °C for 30 s. Quantitative real-time PCR experiments were performed
in triplicate. The transcriptional levels of yncD gene were normalized to the transcripts of a housekeeping gene, rpoD, which served as an internal control. The YncD protein of S. Typhi Ty2 is annotated as a TBDT in NCBI, which was confirmed with our bioinformatics analysis (Supporting check details Information, Appendix S1). To verify whether YncD plays a role in pathogenesis, a yncD deletion mutant was constructed by homologous recombination using a suicide vector pYG4 (Fig. S1). The LD50 of S. Typhi Ty2 and its yncD deletion mutant were measured using the mouse mucin model. As shown in Table 2, the ΔyncD mutant is 1000 times less virulent see more than the wild-type strain. When the pBR322 plasmid carrying the intact yncD gene with its native promoter was transformed into the mutant, the virulence was almost completely restored. These data show that the deletion of the yncD gene results in attenuation. To understand why yncD knockout leads to reduced virulence, we determined the growth characteristics of the LB media-cultured YGC101, YGC102 and YGC103. Fig. S2 shows that the yncD-deleted mutant grows in the LB media as well as the wild-type and the complemented strain. The bacterial
growth curves showed no significant deviation among the three strains. However, the competitive indices of the yncD-deleted mutant in the bacterial competition tests is 0.149 ± 0.093, which indicates a decreased
survival capability of the mutant in vivo compared with that of the wild type. As the yncD deletion mutant was attenuated in the mouse mucin model, we examined its vaccine potential. Among the mice immunized with the yncD deletion mutant, a protection rate of 100% was produced in the groups challenged with 104 and 105 CFU of the wild-type Immune system strain, and a protection rate of 33% was produced in the group challenged with 106 CFU. As all control mice died 2 days after they were challenged with 103 CFU of the wild-type strain, the yncD deletion mutant showed a significant immunoprotective effect (Table 3). The yncD gene was supposedly a target of the PmrAB system by an early in silico analysis (Marchal et al., 2004). The PmrAB regulatory system is required for resistance to the cationic antibiotic polymyxin B and Fe3+-mediated killing. Therefore, the responses of the yncD mutant and the wild-type strain to polymyxin B and Fe3+-mediated killing were assessed. The results showed that no difference exists between the two strains (data not shown). To investigate the regulation pattern of the yncD gene, the yncD promoter region was cloned and inserted into a site before a promoterless egfp gene, which was carried into the pBR322 plasmid.