Salmonella Typhimurium lacking phoBR as a live vaccine candidate against poultry infection.

Salmonella enterica serovar Typhimurium, with a broad-host range, is a predominant cause of non-typhoidal Salmonella infection in humans, and the infectious source is highly associated with food animals, especially poultry. Considering the horizontal transmission of S. Typhimurium from farm animals to humans, vaccination has been strongly recommended in industrial animals. In an effort to eradicate S. Typhimurium in poultry farms, a live candidate vaccine strain lacking the phoBR genes, which encode the PhoB/PhoR two-component regulatory system responsible for cellular phosphate signaling, was evaluated in mice and chickens. Lack of the phoBR genes promoted overgrowth of intracellular Salmonella. However, notably, in BALB/c mouse models, the ΔphoBR mutant showed attenuated virulence and instead, provided protection against infection with virulent Salmonella, thereby clearing out Salmonella in the spleen and liver. Accordingly, immunization with the ΔphoBR mutant increased immunoglobulin (Ig)G and IgM antibody responses and also tended to increase the IgG2a/IgG1 ratio, which is indicative of T helper (Th)1-mediated cellular immunity. In chicken challenge models, immunization with the ΔphoBR mutant significantly boosted the production of IgG and IgM antibodies after the second vaccination. The vaccinated chickens ceased fecal shedding of challenged Salmonella earlier than the non-vaccinated ones and showed no Salmonella in their caecum and ileum. These results demonstrate the potential of the S. Typhimurium ΔphoBR mutant as a vaccine in chickens.

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

Non-typhoidal Salmonella (NTS) is one of the primary causes of foodborne gastroenteritis; occasionally, it causes invasive infection in humans. Because of its broad host range, covering diverse livestock species, foods of animal origin pose a critical threat of NTS contamination. However, there is currently no licensed vaccine against NTS infection. FruR, also known as Cra (catabolite repressor/activator), was initially identified as the transcriptional repressor of the fructose (fru) operon, and then found to activate or repress the transcription of many different genes associated with carbon and energy metabolism. In view of its role as a global regulator, we constructed a live attenuated vaccine candidate, ΔfruR, and evaluated its prophylactic effect against NTS infection in mice. A Salmonella Typhimurium mutant strain lacking fruR was defective in survival inside macrophages and exhibited attenuated virulence in infected mice. Immunization with the ΔfruR mutant stimulated the production of antibodies, including the IgG, IgM, and IgG subclasses, and afforded a protection of 100% to mice against the challenge of lethal infection with a virulent Salmonella strain. The prophylactic effect obtained after ΔfruR immunization was also validated by the absence of signs of hepatosplenomegaly, as these mice had comparable liver and spleen weights in comparison with healthy mice. These results suggest that the ΔfruR mutant strain can be further exploited as a promising vaccine candidate against Salmonella lethal infection.

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

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 ΔyjeK 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 ΔyjeK 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 ΔyjeK 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 ΔyjeK mutant strain. Consequently, vaccination with the ΔyjeK 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 ΔyjeK mutant strain can be exploited as a promising live attenuated NTS vaccine.