Vaccines are a rational and cost-effective means for protecting against infectious diseases in travelers, military personnel, and in endemic developing country populations. Our goal in this proposal is to address several significant vaccine needs: 1) the need for an easy-to-administer (needle-free) and safe oral vaccine vector platform for stable expression and delivery of multiple foreign antigens, that generates long term efficacy following a rapid immunization regimen and which can be distributed without the need for refrigeration; 2) the lack of a licensed vaccine for prevention of morbidity and mortality due o shigellosis; and 3) development of a multivalent oral vaccine that will simultaneously protect against multiple disease agents (i.e. enteric fever plus the major causes of shigellosis). To address these challenges, we exploit the extensive safety record of the existing live, oral, attenuated Salmonella Typhi Ty21a typhoid vaccine by utilizing it as our lead candidate vector to develop a combination oral vaccine that will simultaneously protect against both typhoid fever (with cross-protection against some paratyphoid fevers) and shigellosis. Further, we hypothesize that this vaccine can be formulated to be safe, stable, and highly immunogenic and can be easily administered orally. The current proposal is aimed at creating multivalent vaccine strains Ty21a expressing S. sonnei form 1 O polysaccharide (Ty21a-Ss) and Ty21a-expressing S. flexneri 2a form 1 O polysaccharide (Ty21a-Sf2a). We will also create acid resistant Ty21a-Ss and Ty21a-Sf2a by adding Shigella glutamate decarboxylase (GAD) genes into vaccine candidates and fully characterize each strain genetically and biochemically. Immunogenicity and protective efficacy against S. Typhi (Ty2), S. sonnei (53G) and S. flexneri 2a (2457T) in mice of Ty21a-Ss, Ty21a-Sf2a, Ty21a-Ss-GAD and Ty21a-Sf2a-GAD will be assessed by immunization via intranasal installation of doses followed by mucosal challenge. In Phase II, we will conduct similar work for S. flexneri 3a and 6, finalize a temperature-stable dried product as rapidly dissolvable wafers or tablets, conduct all pre-clinical IND enabling studies, finalize a clinical protocol, and submit an IND. This Phase I project will provide the foundation for the Phase II to complete construction of a quadrivalent anti-shigellosis vaccine that will protect against more than 85% of shigellosis worldwide.