## 1 Introduction  
Salmonellosis caused by non-typhoidal _Salmonella_ (NTS) is a major foodborne gastrointestinal disease worldwide [1, 2]. The Centers for Disease Control and Prevention reported that _Salmonella enterica_ is responsible for over \(1\) million cases of foodborne disease per year [3]. The symptoms of NTS infection, including diarrhea, abdominal cramps, and fever, are generally self-limited in healthy individuals. However, some NTS strains are invasive and enter the bloodstream, causing systemic infections such as septicma, meningitis, and hepatosplenomegaly [4, 5]. Infection with invasive NTS is often lethal in the old and the weak, especially immunocompromised patients. Furthermore, considering the increased prevalence of multiple-drug-resistant NTS strains, invasive NTS poses a serious threat to human health [6, 7]. However, despite the maleficence of invasive NTS, there is no available licensed vaccine for the prevention of invasive NTS infection [8].  
In an effort to develop vaccines against invasive NTS, various types of vaccines, including attenuated vaccines, whole-cell killed vaccines, and subunit vaccines, have been evaluated [9]. Among them, attenuated vaccines have been extensively explored because of their cost-effectiveness and outstanding prophylactic effects against infectious agents. A successful attenuated vaccine should be strongly attenuated regarding virulence, but stimulate a significant humoral and cellular immunity in the host after inoculation.  
There have been several reports about attenuated vaccines against NTS infections [9; 10; 11]. However, these attenuated vaccines are not licensed for humans.  
The aim of this study is to develop a live attenuated vaccine against NTS infection. For the first step towards successful vaccine development, we evaluated the potency of _Salmonella_ strains lacking FruR, SsrAB, or Hfq as live attenuated vaccines. SsrAB is a two-component regulatory system of the _Salmonella_ pathogenicity island-2 (SPI-2), which is a critical virulence determinant that is required for _Salmonella_ survival and replication inside host cells [12; 13]. Hfq was originally identified as host factor I for bacteriophage Q beta replication, and then found to regulate the translation of more than 20% of all _Salmonella_ proteins as a post-transcriptional regulator [14; 15]. FruR, in addition to repressing the fructose (_fru_) operon transcriptionally, is extensively involved in carbon utilization and energy metabolism [16], including glycolysis, the Krebs cycle, the glyoxylate shunt, gluconeogenesis, electron transport, and pentose phosphate pathways [17]. Moreover, _Salmonella_ lacking FruR is attenuated regarding its virulence in animal infection models [18]. Regarding the multifaceted roles of FruR in bacterial virulence regulation, as well as metabolic regulation, _Salmonella_ lacking FruR is an ideal candidate live attenuated vaccine against NTS infection. Here, we evaluated the prophylactic effects of three _Salmonella enterica_ serovar Typhimurium strains lacking FruR, SsrAB, or Hfq against invasive NTS infection. Among these three strains, _Salmonella_ devoid of FruR exhibited significantly decreased virulence accompanied by strong immunogenicity in a mouse model, protecting host animals against NTS challenges. These results suggest that the _$fruR_S. Typhimurium strain is an ideal attenuated vaccine candidate for preventing invasive NTS infections.