Enteric fever is a clinical syndrome caused by Salmonella enterica serovar Typhi and Paratyphi A or B. Recent increases in the frequency of S. Paratyphi A (SPA) in Asia and emergence of antibiotic resistance have complicated treatment and posed a risk to U.S. travelers. In contrast to S. Typhi, there is no vaccine to prevent SPA, a category B priority bioterror threat to U.S. food sources. We have engineered two SPA candidate live oral vaccine strains by deleting the guaBA chromosomal locus (which encodes two enzymes in the distal de novo purine biosynthesis pathway) and also introducing deletions in either c/pX (encoding an ATPase) or dpP (encoding a protease), to create strains CVD 1902 and CVD 1903, respectively. Deleting guaBA has been a promising strategy for attenuating pathogenic S.Typhi. Introducing a deletion in either component of the dpXP complex provides a second, independent attenuating mutation to minimize the risk of a recombination that could theoretically restore the wild type genotype and may also enhance protection by hyper-expression of flagellar protein, a protective antigen that elicits both humoral and cell-mediated immune responses. Salmonella deleted in dpP, dpX, orclpXP have decreased ability to produce systemic infection yet the resultant dpXP mutants remain capable of protecting mice against wild type challenge. We propose to conduct a Phase 1 trial to evaluate the safety, tolerability, and immunogenicity of escalating dosages (107, 108, and 109 CPU) of CVD 1902 and CVD 1903. Three consecutive cohorts (each receiving a higher dosage of vaccine) of 14 healthy subjects 18-45 years of age will be admitted to the CVD Research Isolation Ward for 20 days followed by 4 outpatient visits and a telephone contact at 6 months. Subjects in each cohort will be randomized (double-blind) to receive a single oral inoculation with either CVD 1902 (N=6) or CVD 1903 (N=6) vaccine or placebo (N=2). During their 180-day participation, subjects will be closely observed for clinical response and will donate serial samples of blood and stool to detect colonization with the vaccine strain, to ensure that the strain was eliminated by per-protocol antibiotics, and to evaluate the ability of the vaccines to stimulate relevant mucosal, humoral, and cell-mediated immune responses. The results will be analyzed with the aim of selecting a well-tolerated and immunogenic vaccine strain and dosage for further clinical development.