Salmonella Typhimurium Lacking YjeK as a Candidate Live Attenuated Vaccine Against Invasive Salmonella Infection

Abstract

Non-typhoidal Salmonella (NTS) causes gastrointestinal infection, which is commonly self-limiting in healthy humans but may lead to invasive infection at extraintestinal sites, leading to bacteremia and focal systemic infections in the immunocompromised. However, a prophylactic vaccine against invasive NTS has not yet been developed. In this work, we explored the potential of a yjeK mutant strain as a live attenuated vaccine against invasive NTS infection. YjeK in combination with YjeA is required for the post-translational modification of elongation factor P (EF-P), which is critical for bacterial protein synthesis. Therefore, malfunction of YjeK and YjeA-mediated EF-P activation might extensively influence protein expression during Salmonella infection. Salmonella lacking YjeK showed substantial alterations in bacterial motility, antibiotics resistance, and virulence. Interestingly, deletion of the yjeK gene increased the expression levels of Salmonella pathogenicity island (SPI)-1 genes but decreased the transcription levels of SPI-2 genes, thereby influencing bacterial invasion and survival abilities in contact with host cells. In a mouse model, the yjeK mutant strain alleviated the levels of splenomegaly and bacterial burdens in the spleen and liver in comparison with the wild-type strain. However, mice immunized with the yjeK mutant displayed increased Th1-and Th2-mediated immune responses at 28 days post-infection, promoting cytokines and antibodies production. Notably, the Th2-associated antibody response was highly induced by administration of the yjeK mutant strain. Consequently, vaccination with the yjeK mutant strain protected 100% of the mice against challenge with lethal invasive Salmonella and significantly relieved bacterial burdens in the organs. Collectively, these results suggest that the yjeK mutant strain can be exploited as a promising live attenuated NTS vaccine.

Evaluation of Salmonella Typhimurium Lacking fruR, ssrAB, or hfq as a Prophylactic Vaccine against Salmonella Lethal Infection

“…Primers used in the diagnostic PCR are listed in Table S2. ST2173 used in the challenge experiments was constructed by transforming ST14028 with the pBBR1-MCS4 plasmid as described previously [23]. Salmonella strains were cultured in Luria-Bertani (LB) medium at 37 • C with antibiotics when required, unless otherwise specified.…”

“…Salmonella survival inside macrophages was assayed as described in previous studies [19,23]. Briefly, murine macrophage RAW264.7 cells were cultured in Dulbecco's Modified Eagle's Medium (DMEM) supplemented with 10% fetal bovine serum and seeded in 24-well culture plates at a cell density of 2 × 10 5 cells/well, respectively.…”

“…Bacterial virulence was evaluated in vivo using the half-maximal lethal dose (LD 50 ) estimation [23,27]. Six-week-old female BALB/c mice were purchased from Orient Bio Inc. (Seongnam, Korea) and acclimated for 7 days prior to infection.…”

“…The specific antibody responses of IgG, IgM, and IgG subclasses (IgG1 and IgG2a) to S. Typhimurium OMPs were assessed using ELISA, as detailed previously [23,28]. Using triplicate sera from immunized and non-immunized mice, the optimal concentrations of serum and enzyme conjugates were determined based on the method of the checkerboard test [29,30].…”

Secretory System Components as Potential Prophylactic Targets for Bacterial Pathogens

“…Protein synthesis pathways interference was used in another attempt at live vaccine. Park et al [285] used a ∆yjeK mutant deficient in the protein synthesis elongation step. BALB/c mice were injected with 10 4 CFU/dose of the S. enterica Typhimurium 1120 strain via the i.p.…”